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Cartridge Firing Dynamics

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 13:36
303Guy View Drop Down
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 I have a few questions for  Graysteel and Dale Clifford and anyone who can help enlighten me.

 

 The way the powder behaves when it ignites - I think you know what I am getting at here.  Primer flash, flame spread through the charge, pressure rise, bullet acceleration, rifling resistance, effect of case neck grip, and so on.

 

The way the cartridge case behaves on firing.  Case grip, case expansion, case slip on chamber walls, case head impact on bolt face, rate and magnitude of load applied to bolt face and so on.

 

I am also wondering how the above may affect accuracy.  I have noticed on some of my recovered bullets that the bullet did not enter the rifling perfectly aligned with the bore.  I solved that by seating the bullet in an un-sized case using a soft paper cup dipped in molten bullet lube.  I have not tested that one on the range.  Initially, the bullets yawed and tumbled.  Those were 55gr BTHP in a 1-in-16 twist hornet, with a long bullet jump.  The same rifle stabilizes 55gr Spire Points with the paper cup technique.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 15:21
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I would be more than happy to offer any opinion I have on those issue. With the usual disclaimer that half of what I believe today will probably change by tomorrow.

I feel pretty comfortable with answering questions on the behavior of powders. I am less qualified to comment on the engineering of the interface between the brass and the chamber, but I am happy to share what I know.

I am not really sure if you have specific questions, but I can start with a quick bit of info on powder. Modern smokeless powders are derived from High explosives that have been modified to be difficult to detonate. In the normal function of a firearm they deflagrate (burn) rather detonate (explode). That is to say they behave as a low explosive. The reason this is important is because things that deflagrate tend to do so much more irregularly than things that detonate.

Think of it as trying to burn a bunch of small sticks or pellets. How do you get them to burn the same way every time? This is where the primer comes in. In an ideal world when you set off the primer (which detonates) the hot gases from the primer would instantly fill the entire shell casing and begin the combustion of every single grain of power at the exact same time.

Of course, we don't live in the ideal world. In reality, the hot gases from the primer vary in intensity depending on the quality of the primers. This in turn affects how quickly and how evenly the grains of power burn. A primer that doesn't bathe all the power in an evenly hot flame won't burn the powder the same way each time. This means the pressure curve will differ from shot to shot. Thus the bullet won't accelerate the same each time and won't leave the muzzle the way same each time. The pressure curve from the burning powder also affects the harmonics of the barrel, etc. etc. So you can see why good consistent primers are important.

Next, is the shape the shell casing itself. Long narrow shell casings require the hot gases to propagate all the way through the column of power before they can start burning the grains or powder at the end of the column. The longer the column the more difficult it is to start all the grains at the same time. The powder will also lay differently in each cartridge and therefore change the way everything starts burning. This is also an issue if you have a lot of empty space in your shell casing, the powder will lay very differently in each shell.

No matter the shape of the shell casing is you will have to deal with this issue of irregular ignition to some degree, but one can minimize this effect by creating a shell casing that allows the primer to ignite things more consistently. This is part of the reason why short fat shell casings are more accurate in general. The primer is located closer to the geometric center of the power charge. This tends to reduce irregularities in the power initiation.

The short fat shape also tends to even out the burning of the power grains themselves. This is because each grain averages less distance from all the other grains of power, which tends to even out things such a heat transfer etc.

Also, the geometry (shape) of the grains of power themselves influences how consistantly the powder burns.

There are also some mechanics about the shape of brass related to how consistently the gases escape the shell casing and I am not very familiar with those. Compressible fluid dynamics is just not a subject I understand very well. Perhaps one of the other folks (Such as Dale hint hint) might be able to shed some light on this part of the system.

I am going to leave it there for the moment, perhaps I could get you to elaborate on what areas you are interested in?





Edited by Graysteel
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 17:21
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the amt of energy needed to break, or make a chemical bond is called the activation energy, and depending on this being neg or pos. determines if the reaction is exothermic, endothermic, what the enthalpy of reaction,entropy and heat of reaction will be. propellants contain both the oxidizer and reducing agents in a sealed container. As the chemical bonds are broken and the combustion products are formed the molecules collide faster and faster as the temp and pressure go up exp. which results in the fast ignition times of propellants as opposed to the linear decompostion of black powder type or ammonium sulfate types. (and another reason you should'nt be playin with duplex loads) As the gas expands, the flame front moves in the line of least resistence at a speed a property of the media. up to 6000 fps for propellants, and around 10,000 for HE (why they are called High, high speed of flame front). propellants can contain up to 10% nitro, and work well for angina and headaches, and why some states such as Idaho restrict there sales to guns only, and no headaches. (sorry got side tracted).

The physical property of combustion is an evolutionary equation of the Navier-Stokes type or partial differential in 3 dimensions and is hard to work with, but the mechanics of guns is pretty well established and the properties can be controlled somewhat by particle size, rate inhibitors, and container size.

As the molecules heat up, and collide the vibrations create, ( and also the shock wave from the primer itself) waves that can interact with the inside walls of the container. Rounded shoulders on shorter cases can reflect this wave back toward and deeper to the center causing more efficient combustion and an advantage for these types of cases, the straighter the wall of the case the more the primer is left to blow the charge out, which it will do along with the bullet, just not as fast as we would like. Which brings up straight wall cases such as 300 h&h and 22 hornet, (almost straight anyway).

Have a cz hornet and a tauraus 10" raging hornet, . spire pts have the center of mass back quite a bit more than hpbt

and in the revolver the 35 vmax hornady is the most accurate no doubt to the center of mass being back. while the cz like the 40 gr. vmax, a relativley long bullet compared to the 55 gr spire pt. both prefer that the neck only be rezsized about 1/2 way down.

crimping and neck tension obviously hold the bullet and allow the combustion to go to completion.

it would be a pretty long post if you want to go into the lbs. of force at each step in the process as this is a thermodynamic process ---so do you have a specific question it would be easier

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 18:46
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Seems to me that your are seeking a reloading answer to a chamber problem and a twist problem.  Given your address that is most reasonable train of thought.  The longer bullet you are using wants a faster twist, say a 1 in 12 or 1 in 9, 8.5, 8 or 7 and a bit more speed.  They are most common for the .223 and .22-250.  The 52 and 53 grain stuff is common for the hornet and does ok in the 1 in 9 barrels. 

 

It sounds like the chamber maybe off center.  I'm not suprised at that.  There are lots of reason chambers are cut loose.  If the barrel is not turned and the reamer held straight that is gonna happen. Normally stuff gets chambered the other way around, which creates the problem you describe. Set the bullet out as far as you can (That and anything you can get from the local witch doctor.) will help. Neck size the cases only, to get a tighter chamber fit.  Otherwise, you would need to pull the barrel and get it turned down a thread or so and recut the chamber in the manner I descibed. 

 

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 21:42
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Greysteel, thanks for that.  (Are you by any chance a lecturer?  You make it all seem so clear)!  Indeed, I don't have any 'specific' questions but I want to know all about what is happening in my reloads.  I have many theories, which I cannot test.  I could probably ask you questions for hours!  You have touched on areas I have wondered about for years.  Thanks again.

Dale, you wisely point out the risks involved with duplex loads.  Rest assured, I did not jump in totally blind.  (Nor will I play with it again - but it was fun).   I did manage to reduce pressures without too much loss of performance.

With regards to neck tension, some folks use a lot of crimp and a small pistol primer in their hornets to get best accuracy (with Lil'Gun).  I am using AR2205 (H4227), which, I'm sure, has a very different burning characteristic.  I don't neck size but I still seem to get consistent ignition.  My thinking is that the time lapse from onset of ignition to bullet leaving the case and entering the bore, must be pretty short so the influence of case grip could well be approaching zero since the rising pressure would expand the case to release the bullet before any appreciable bullet movement has taken place.  Which might explain why many rifles seem to shoot better with a small jump.

On the question of flame front velocity - how does it work with a propellant?  How does the rise in pressure and temperature work when the propellant has to decompose to release its O2 (and reducing agents)?

Silver, I do have a problem with a lousy chamber - it is rough and large but does not appear to be out of line with the bore.  I have managed to get some pretty respectable accuracy results with this rifle and I cannot measure any runout on my reloaded cartridges (unless you can call 0.02 mm at the bullet tip, runout - some would say "eight tenths of a thou").  I have loaded 'long' but the magazine restricts me so I am kind of stuck with the jump.  Since my bore is damaged anyway, I was considering re-barrelling and was wondering what kind of twist rate would suite me best.  I would probably get a barrel blank and cut the chamber myself - in a lathe.  My barrel is pinned - not threaded but has a pillar mount under the rear iron sight.  I wanted to see what the rifle could do first and was astounded that it shoots quite well.  Unfortunately, I have not been able to do proper grouping trials since the initial sighting in at 100 m and 200 m.  Then I got 1.1 MOA with 45gr bullets.  Subsequently, I managed ± 1/2 MAO with paper sabot seated Sierra 50gr Blitz. (One group at 45 m does not define accuracy).  The one test I want to do real badly is to try out those boat tails again now that I can get them into the bore straight.  I don't really expect them to stabilize.

 

On the question of case grip, I have a theory I want to test.  It is that if there is too much friction between the case and chamber walls, it will result in sudden loads being applied to the bolt face when the case does finally 'let go' of the chamber walls or the brass yields, just above the web, when the pressure gets high enough.  If the case has limited grip, it will go elastic and 'creep' off the chamber walls, allowing the head to rest firmly against the bolt face, before peak pressures occur.  The only observations I have been able to make is that a case that is fired dry in a rough chamber, stretches more - far more - and even fails, at much lower pressures than a case that is lightly lubricated.  I can see on my cases that the area above the web has moved against the chamber walls but has not moved at the neck area.  The reamer grooves are impressed into the case at the neck area while they are 'rubbed' at the web area.  No case stretch occurs (but if it were to, I would reduce the load and disassemble any more cartridges of that loading).

 

This topic is extremely interesting to me - thanks again folks.



Edited by 303Guy
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/26/2007 at 23:37
Graysteel View Drop Down
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No, I don't teach for a living. For a short while after I came out of the service I did a bit of technical writing on energetic materials. I know you directed the question of front velocity to Dale and I don't mean to step on toes, but I thought I might offer a few thoughts.

With pelletized materials such as smokeless powder, the 'front' does not entirely describe the method of propagation. The primer actually pressurizes the entire shell casing before significant initiation has occurred. The reason for this is the primer is really a High explosive.

High explosives do not burn per say - They detonate. Detonation is a bit of a complex process. But in layman's terms, detonation is the chemical decomposition of a material along a shock wave. Said another way, a shock wave travels through the material and as the wave front bumps the molecules together it imparts enough energy to split them apart. This quality is what causes high explosive to always detonate at a specific rate. That rate is the speed of sound in the material in question.

When your firing pin sets off the primer the explosive in the primer decomposes at the detonation velocity of the primer material. This rate is much faster than low explosives such a propellant deflagrate (burn). The hot gases then find their way through the powder in the manner that Dale said, by the path of least resistance. The path of least resistance is the spaces between the grains of powder.

This wave of hot gases is what starts the powder deflagrating.  The order in which this happens is made a bit more complicated because when the primer first detonates the pressure wave forces the power towards the top of the case and out to the sides and the powder then bounces around more as the pressure tries to equalize.

Now is the fun part. Reaction kinetics. Low explosives such a smokeless power have the quality that they deflagrate faster as pressure increases. So... the more pressure the faster they burn, the faster they burn the more pressure at an increasing rate. It becomes a run away reaction. This is also why a 'little more' powder can make a lot more pressure.

I mentioned the geometry of the powder in my previous post, here is where it comes in. The hot gases from the primer are are all around the powder grains the grains start to deflagrate. Well, they generally only deflagrate on the surface of the grains, as the grains get smaller there is less surface area and so the rate of pressure increase drops off. There have been powders made with toroid (donut) shaped grains to keep a more consistent velocity, but in most cases what you want with a rifle is for rate of deflagration to level off to give the bullet time to start moving down the barrel.

I am happy to ramble more (most people are smart enough not to listen to me) but I don't want to over do it at one time.


Edited by Graysteel
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/27/2007 at 00:28
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Thank you very much GreysteelThis is fascinating!  So... in the test conducted by the US in which they set off - if memory serves - 500 tons of high explosive to simulate a nuclear explosion, would they have used multiple detonation points throughout or on the surface or just a single, central one?   I digress.  You got to where the bullet starts moving ......  

 

Oh... this would be where my other question fits in - the one about case grip on chamber walls.  My guess is that at this point the case is firmly gripped by the chamber walls in a forward position, if there is any headspace clearance at all.

 

There are going to be a number of spin-off questions too - like how free space effects the rate of pressure rise etc.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/27/2007 at 10:37
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303Guy Check out this link

http://www.varmintal.com/arelo.htm

I will paste an excerpt.

From Varmint Al's Page:

_______________________________________________

POLISHING MY CHAMBERS.... When I got my 223 Ackley Improved Virgin Valley (no longer in business) barrel, it had a very rough chamber. The fired brass had marks I could see where it had been gripped by the chamber and then slid backwards a tiny bit until stopped by the bolt face. It looked almost sandblasted. There were protruding primers on some of the fired rounds. When the firing pin hits the primer, it pushes the primer and the new case forward in the chamber. When ignition occurs, the pressure expands the walls of the brass. The rough chamber, with its high friction, grabs the case at the forward position and the gas pressure first pushes the primer back against the bolt face. As the pressure builds, the brass case is stretched until the case head is pushed back against the bolt or the case supports the total load. If the case head is pushed against the bolt face, it would produce a flat primer, but flush. If the load is light, the case does not stretch so much and leaves a gap between the case head and the bolt face, but the primer is still against the bolt face. This produces the protruding primers.

CONTROVERSY OR GOOD ENGINEERING.... Now some controversy. I polish my chambers with Flitz. I don't want excessive friction between the brass and the chamber wall. Some are going to argue with this, but I have thought long about this. I have tried it and it works and is good engineering. I polished the chamber in my new Virgin Valley (no longer in business) barrel and it has made quite an improvement. With a polished chamber, the friction coefficient is much less between the brass and the polished stainless steel chamber. The pressure is able to force the case head against the bolt face before the case walls grab the chamber. First, this lets the primer (on the first shot with new brass) protrude, but be immediately reseated in the primer pocket as the case head is pushed back. The brass does not stretch nearly as much as it would in a rough chamber. Now some will say that a polished chamber will increase the force of the bolt face (frame face on the Encore) and that is bad. WRONG! That is faulty logic. The force is increased on the bolt face, but that is where it belongs. As all modern rifles, the barrel and frame of the Encore are designed for strength. They are strong enough to support large diameter Magnum-belted calibers. The force from the much smaller area of the .223 case head is easily supported by the frame face. The brass case is designed to act as a bladder and encase the gas pressure. Trying to use the weaker brass to lighten the load on the action and bolt face of a rifle by having the brass grip the chamber is analogous to using a car's radiator to protect the bumper in a front-end collision. A polished chamber minimizes case stretching, reduces case head separation, and increases case life. More information on chamber finish here. To polish a chamber, I put Flitz on a cotton bore mop and a piece of cleaning rod long enough to be held in a drill motor. I polish for about 30 seconds or more at a 300 to 600 rpm speed. Sometimes I have to wrap a paper towel around the swab and put Flitz on it to get a good fit. Be sure that the end of the bore mop's metal part is much smaller than the bore and covered so it will not damage the rifling in front of the chamber.
_______________________________________________


He also completed a much more detailed analysis at the following link:

http://www.varmintal.com/a243z.htm


I am not ducking your question, I just couldn't ever give as good of an answer.

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If your going to rebarrel go with the k hornet better accuracy and performance. the best result is when the case is the same as the chamber, (neck sized or partial resized). It sounds like your already getting pretty good accuracy with what you have to work with. Ordinarily the firing pin will drive the case forward until the case shoulder touches as or before the primer actually starts ignition. On the hornet which is rimmed it headspaces on the rim and is not a good example of usual case dynamics. Hornet brass is also notoriously thin in the mout area so further comparison becomes difficult. Case crimp can have substantial effect on ignition consitency as shown by, 296 squib loads in 357 and up class cartridges, and has more importance the straighter the case wall. would suggest you go to a slightly faster 410 poweder H110, 296 maybe even HS6. Usually loads shoot better with no gap as this causes a start stop start situation, but can cause two momumental problems, pulling the bullet out and dumping powder in the action and not enough camming to close the action (occurs in lever actions and one of the reasons they are crimped).

When gases under go reactions the rate constant is 10s of magnitudes greater than even reactions in fluids. The breaking of the n-o bond is endothermic (absorbs) while the c-o is exothermic (releases).

the flame front is a work concept, it is the same for propellant, HE, and thermonuclear- its what you have to work with after the molecules stop changin.

analysis of the difference between the reaction rate kinetics of propllants and HE is a thermodynmics approach, (more accurately statistical mechanics).

getting abunch of stuff to go at the same time is very hard. something like krypton triggers must be used and the charge itself must be correctly shaped. just be glad it doesn't do this in guns.

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man you guys are giving me headache trying to read this.  
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Greatly appreciated Folks.  This is fascinating stuff

 

Greysteel,  it seems I was on the right track after all.  What I have seen in other rifles, is what appears to be signs of excess pressure, when in fact it is signs of excess case stress caused by rough chambers and dry cases.  This could translate into excess bolt face thrust - probably is, due to the suddenly applied load factor.  Varmint Al has explained the case grip/case head seating exactly the way I was thinking.

 

There are so many things I would like to ask!  I was wondering how that hornet case shape might influence the ignition.  I wondered whether the primer pressure is enough to drive the bullet into the bore before the initial pressure build up, which is why neck crimping is used.  In a stronger case, the primer might not be able to release the bullet.  It would all be to do with timing.  The powder needs to burn before the bullet can engage the rifling to avoid the stop-start motion.  But if the bullet is already resting on the lands, then that problem would disappear but a new one would emerge - bullet inertia plus engraving force at the same time.  This would cause more delay in volumetric expansion leading to higher pressures.  Not a problem if one designs the loads for it.  In my case, I have a long jump to the lands so maybe pressure build up is quick enough to drive the bullet after it has been moved by the primer and before it hits the lands. 

 

Dale, on the K-hornet.  I was thinking along the lines of a modified standard hornet with the case taper being extended further forward to form a 30° shoulder.  The idea is to avoid the need to fire-form cases.  Another Idea is to change the normal K-hornet profile to a hybrid.  Instead of tapering the chamber from the web to the shoulder, as per K-hornet, reduce the K-hornet dimensions slightly so that the K taper begins just forward of the web, with the standard hornet taper from breach face to that point.  That way, a standard case will be supported by the web and by the neck.  I would not go for an extremely tight neck as I want to use ‘out of the box’ cases.  (Now if I could have interchangeable barrels, I could go with a hunting barrel and a cast bullet barrel.  And if the action was stronger I could even have a 221 fireball barrel – dreams, just dreams).

 

 

Can anyone suggest why a rust damaged barrel should be able to shoot straight?

 

I have too many questions, don’t I?

 

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Originally posted by pyro6999 pyro6999 wrote:

man you guys are giving me headache trying to read this.  

 

I know what you mean pyro  .  I have to read it more than once to get it.  These folks are so knowlegeable - I just love reading their stuff!  They should compile a book.

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Originally posted by <strong>silver</strong> silver wrote:

.......

It sounds like the chamber maybe off center.  I'm not suprised at that.  There are lots of reason chambers are cut loose.  If the barrel is not turned and the reamer held straight that is gonna happen. Normally stuff gets chambered the other way around, which creates the problem you describe. Set the bullet out as far as you can (That and anything you can get from the local witch doctor.) will help. Neck size the cases only, to get a tighter chamber fit.  .........

 

Thanks for your post Silver.

 

I did try this initially but the throat is so long that I cannot modify the magazine to accommodate the OAL.  I have thought of using a round nose type bullet to get the ogive closer to the lands but have since found that the Sierra 50gr Blitz, with it's shallow ogive and long jump, shoot real well.  So do Super-X 218 Bee 46gr RN hollow point bullets.  I have often wondered whether my 303 has an eccentric chamber for the very reason you describe.  I had a #4 barrel fitted to my mk I which required setting the barrel back to align the extractor groove with the bolt.  This tightened the chamber considerably but I have never checked for chamber concentricity in either rifle.  Perhaps I should.  It's just not so easy with the barrel fitted.  It has been said. 'though, that run-out is only a factor when you get to sub-MOA levels of accuracy.  Since I have very little run-out, it may not be too big an issue for me - yet.

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had a cetme (springfield knock off ) that had such a tight and rough chamber, i had to lubrisize every 5th case with RCBS lube to make the gun function, and of course the bolt velocity showed it. the primer can definitely blow the powder and bullet into the gun before the pressure curve has had enough time on the powder. personally I't just go straight to the fireball, a great round in its own right.

damaged, rusted and pitted barrels can and do shoot great, its the riflings that are the problem, and the fact they foul worse and faster than normal.

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Dale already answered the question about the primer being fully capable of moving the bullet forward before the powder completely initiates. And yes, you are correct that the combination of inertia and engraving resistance will tend to increase the rate of increase during the first part of the pressure curve. In fact, I think you will find quite a few warnings about increasing AOL on a round that is already at it's maximum pressure.

To be honest I have not idea why neck crimping is used in that round. But, I wouldn't think it is related to pressure curve.
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the chamber concentricity is not as important as the neck , although if you have one usually the other is present. turn the outside of your necks with a sinclair or equiv. neck trimmer and it will become immediately apparent.

some early hornets were at one time chambered in lever actions and the crimping would keep the bullet from set back.

long jump doesn't preclude accuracy, look at the free bore in weatherbys, which by local reports are up with the best. (should say that I had a weatherby and it was extremely accurate just had some other problems also I freebore all my 9 mm pistols so as to shoot 9 major without effects on accuracy) If one of riflings has an edge that is chipped or wearing uneven this will also tip the bullet on entry and cause unequal engraving

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Thanks for the replies.  I must say, I do not seem to have a problem with fouling and that puzzles me too.  I cannot see any sign of copper in the bore.  I have recovered bullets that show no sign of copper wiping.  The same with cast bullets.  Maybe someone polished the bore after the rust damage.

 

These folks who crimp their hornets do so for accuracy and they do it in conjunction with small pistol primers and Lil'Gun.  Well, OK - you folks have explained that there is a LOT going on in a cartridge upon firing.

 

I have measured loaded rounds for neck diameter variation and have found 'high spots'.  The hornet case is so thin already but maybe I should consider neck trimming.  I could try one or two on a turned mandrel in a lathe to see how bad they might be.

 

I have thought about the fireball.  It's just that the chamber pressures for that round are a bit high for my Anschutz - not to mention the larger internal case base area.  It might be too easy to overload the bolt lugs.  Now if there was a 6mm hornet ......   I wonder.......  with K-hornet case taper ......  it'll drive a 60gr bullet to about 2500 fps ?....  that would put it into small deer territory ..... (It handles ferrel goats just fine as it is, using 50gr bullets)!

 

I'm guessing the 9 major requires a minimum recoil level and the free-bore allows higher loading levels at safe pressures?  The same principle as Weatherby's?

 

(Why shouldn't there be a 6mm hornet.  They have a 25 hornet)?

 

 

 

 

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Originally posted by Dale Clifford Dale Clifford wrote:

had a cetme (springfield knock off ) that had such a tight and rough chamber, i had to lubrisize every 5th case with RCBS lube to make the gun function, and of course the bolt velocity showed it..........

 

Dale, what do you mean by 'bolt velocity'?

 

 

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The HK roller locking mechanism uses spent gases to aid in cartridge extraction by venting them through fluting in the chamber. If the chamber is tight, the vented gasses have less space to expand and pressure increases. Hence the bolt velocity increases.

This may differ from Dale' experience but I have been shooting a 91 for thirty years. My chamber is not tight and if I use powders that are too slow or experience any bullet set back the bolt definitely gets slammed back. Once you exceed the force required to open the roller cams buy "x amount" the story is over. Any excess gets spent on the bolt and hammers the receiver.

 

Doug

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/29/2007 at 10:31
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as tahqua pts. out the cetmes  are really sensitive to reloading and spring rates. if you thiink loading for hornets is a bee nest (couldn't resist) try cetmes-- atrue test of dedication.
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Thanks for that.  I had that problem with my humble mini-14.  It was more accurate with slow powders (possibly barrel harmonics?) but they slammed that bolt back hard enough to bounce the cases into a jam against the scope mount - it was a spring eject.  I eventually installed a smaller gas port insert.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/30/2007 at 01:03
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Originally posted by Graysteel Graysteel wrote:

........
To be honest I have no idea why neck crimping is used in that round. But, I wouldn't think it is related to pressure curve.

 

OK.  I have thought about this one and based on what you folks have explained, I would hazard a guess that what is happening here is that a slower igniting powder is being used and to get consistent ignition, a lower power primer together with more neck grip (crimping), the bullet is not dislodged by the primer, thus allowing the pressure curve to develop.  Does that sound probable? 

Another, less likely scenario is that crimping actually bulges the case at the shoulder/body junction or at the base of the neck, which aligns the case better. 

A third (and slightly more likely than the second) scenario is that the crimp does not expand much, acting as a bullet guide as it enters the bore.  After all, I am using a waxed, paper cup as a bullet guide.

 



Edited by 303Guy
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This picture shows the rough chamber impressions on the lower two cases very clearly.  The top case shows a bright ring closer to the neck.  This ring has a corresponding depression ring on the inside of the case, visible only in bright sunlight.  Neck scratches are also visible.  This is damage from another rifle.  This particular case has been reloaded by me, quite a few times now and has just produced my most powerful load - A 56gr cast bullet in front of an undisclosed amount of powder.  The case shows less pressure than the two beneath it and has zero elongation.  The centre case has 'sliding' marks near the rear but these are not visible in the photo.

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A 55gr Hornady Spire Point and a 50gr Sierra Blitz Med Vel recovered from the firing tube.  They had been fired through a medium consisting of wet wool in a beer can, then captured in wool and cloth.  This being from the rust damaged, hornet bore.



Edited by 303Guy
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I think Dale commented on the reason for the crimping...

 

"some early hornets were at one time chambered in lever actions and the crimping would keep the bullet from set back"

 

A slower powder is not specifically more consistant, it simply reacts more slowly giving a smoother (less peaked) pressure curve. Generally, the driving force behind powder selection has to do with tuning around peak pressure.

 

I have to say I am curious about your bullet catcher.  :)

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