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Barrel Lengths, Rifle Design

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 13:49
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I'm new to scopes and shooting, and the bulk of my information comes from British hardbacks dating back to the late 80s and early 90s. 

Back then, my concept of "proper" barrel lengths the following rounds was 20" for the 5.56 x 45mm, 16" for 7.62 x 39mm, and 20 - 24" for 7.62 x 51 to 63mm.

Now I'm seeing a rash of rifles with enough rails to accommodate a mass transit system and barrel lengths ~ 10 to 16" for both small and medium calibers; has something changed significantly with powder composition and barrel metallurgy (and peoples' hearing) that makes these new dimensions viable?

My mindset's still stuck at preserving barrel length as much as possible.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 13:59
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Powders have changed in a huge way since the 80s.

That said, shorter barrels have their advantages and disadvantages, just like long barrels.

"The right tool for the job" is the best description, that and people want tacticool gear now, with the zombies coming and all. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 14:08
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What's changed significantly from the late 80s?

Then, they were still trying to keep 20" barrel lengths for the 5.56 x 45mm round.

Not sure about this, but the AUG, FAMAS (~ 18"?), and IW still have 20" barrels.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 14:18
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A shorter barrel will typically produce less velocity, and have more muzzle blast.  But for some needs that extra velocity is not really an issue.  For long range shooting it can definitely help, but for short and mid range sometimes a shorter barrel is a plus for the handling and weight factor.

I am sure powders have changed some as well.

I am becoming a fan or shorter barrels just because I like the light weight.  All my ARs have 16" barrels my precision .308 has an 18" barrel.  I honestly prefer them that way. 

For hunting, I just don't think a animal is going to know the diff between 2000 fps or 1900 fps because of a shorter barrel.  It is just not going to matter. 


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 14:27
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I think 18" is the perfect "compromise" length for reasonable velocity and reasonable compactness.  Plus, I prefer the mid length gas system so it isn't over-gassing the bolt carrier assy.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 14:31
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Originally posted by RifleDude RifleDude wrote:

Plus, I prefer the mid length gas system so it isn't over-gassing the bolt carrier assy.


Sorry to change the topics a bit, but I've really started to wonder how much of an issue over gassing really is.  I have a couple of both mid length and carbine length.  With my one carbine rifle I have over 15,000 rounds through it and have had zero issues with the over gassing that every one is always so concerned about.  Same bolt carrier, same gas key, etc.  It has just not caused me a bit of trouble.  I guess I could see an issue with a full auto with tons of shooting, but with civilian semi autos I just don't think there's as much of an issue as everyone talks about on the net.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 14:44
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There are many, many powder options now, far more than there were in the 80s.  Previously there were "slow" and "fast" and not much else, now there is a huge range of burn rates and pressure curves.

All the advances in powder make it possible to get more from less. Likewise, barreling has changed (polygonal, etc) leading to higher velocities from the same barrel length.


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 15:30
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I was previously considering a SIG Sauer 716 Patrol / Precision Marksman.

I guess my next question would be, given the above - what would be an optimal barrel length / bullet weight for a man - sized silhouette out to ~ 600 - 800 yards?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 15:31
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Originally posted by supertool73 supertool73 wrote:

Originally posted by RifleDude RifleDude wrote:

Plus, I prefer the mid length gas system so it isn't over-gassing the bolt carrier assy.


Sorry to change the topics a bit, but I've really started to wonder how much of an issue over gassing really is.  I have a couple of both mid length and carbine length.  With my one carbine rifle I have over 15,000 rounds through it and have had zero issues with the over gassing that every one is always so concerned about.  Same bolt carrier, same gas key, etc.  It has just not caused me a bit of trouble.  I guess I could see an issue with a full auto with tons of shooting, but with civilian semi autos I just don't think there's as much of an issue as everyone talks about on the net.


It's actually fairly common, ST, and a buddy of mine has one that exhibits the classic "over-gassing" traits.  If you see swipes, shaved brass, upturned burrs on the case heads where the ejector sits, this is evidence of over-gassing.  The issue is related to not just the distance between the gas block and the muzzle, but also buffer spring resistance and bolt carrier mass.  If the duration of gas is too long between the time the bullet passes the gas port on the barrel and bullet exit from the muzzle, the bolt carrier moves rearward enough to cause the bolt head to start camming open before pressure fully drops and the brass has shrunk from the chamber walls.  This causes the swipe marks on the case head from the bolt rotating while under max pressure.  If you google "AR bolt head swipe," you'll see examples of this.  The shorter the distance between the gas port and muzzle, the heavier the bolt carrier, and the stiffer the buffer spring you have, the less likely you'll have this problem.  Higher pressure loads also have more likelihood of over-gassing.  The other issue is it batters the inside rear of the upper receiver and stresses the bolt head, reducing its potential life.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 15:34
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Are pistons the way to go these days then? 

I've been eye balling an Adams Arms upper......

Oh man.....slippery slope ahead!! 


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I don't particularly care for piston guns personally, but I'm fairly new to the AR scene, despite being a lifelong gun nut.  Piston systems add moving parts above the barrel that generates vibrations that theoretically has some affect on potential accuracy.  It adds some additional weight.  The piston rod exerts off-axis cantilevering force on the bolt carrier that may cause wear in the lower portion of your upper receiver because the rearward force is way above bolt centerline.  In addition, unlike a simple gas tube, there's no standardization of piston system parts, so if the company providing them ever goes TU, you either have to get parts custom made by a machine shop or retrofit your rifle for something else.  Then, there's the additional cost and mechanical complexity.

That being said, the piston systems I've seen do allow you to adjust the amount of gas bleed, so you should be able to prevent the case head swipe problem.  It no doubt permits you to maintain a cooler running, cleaner action since gas and carbon deposits don't get dumped into the action.

Like everything else -- advantages and disadvantages.

Give me a simple gas tube.  I can always get replacement parts.  I'll just have to do a bit more cleaning than the piston guys.  But, I can understand and appreciate the appeal of the piston system.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 15:54
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My one carbine AR must be setup just right then.  I just have not noticed any of those issues you listed Ted. 

I have an adams arms piston system.  I really like it.  I have not cleaned it in about 2000 rounds and the carrier still looks completely clean.  (I have wiped it down with a towel because it gets dusty from being on my wheeler)  I can dump 2 mags and pull out the carrier and it is completely cool.  I have not really noticed any bad carrier tilt either.  It wore a little at first on the buffer tube, but it has seemed to smooth out and it fine now.

I have about 300 rounds through it with my suppressor as well and it is still staying clean.  I do have to actually clean the gas block though where the gas adjustment valve is.  But that is super easy to clean.

With my suppressor on my 6.8, 50 rounds and it is so dirty it stops functioning.  So for a suppressor setup they are much better in my limited experience with the suppressor. 

It does add noticeable weight to the end of the barrel though.  Just an FYI

Mine still shoots about 1/2 MOA with match ammo.  I honestly did not notice in effect on accuracy one way or the other. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 15:58
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RC, you got any comments for the piston systems?? 

(I got a feeling RC is going to say they are a "solution to a problem that doesn't exist".)  Wink
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:00
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Hm.

Won't be getting any answers for barrel length, the way things are looking...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:02
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I like the one I have and will probably buy another one for my 6.8 at some point.  But the only reason is because I want to use my suppressor on that gun and it just flat out works much better with the suppressor. 

I have never had any issues with the DI systems.  So I will continue to use them as well, the extra cost it quite a bit.  If you were doing a brand new build from scratch the difference in cost it not really that much.  Less than $100 IIRC, so not a big deal.  But to convert one does cost quite a bit.  But that way you do have the DI parts laying around incase it ever fails. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:03
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I apologize for high jacking your thread. 

Carry on...
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:06
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Originally posted by supertool73 supertool73 wrote:

My one carbine AR must be setup just right then.  I just have not noticed any of those issues you listed Ted. 


It's not a given that you'll have a problem with a carbine rifle.  In fact, most the time, it won't be a problem.  It's all a timing deal.

Good comments on the AA system and good point about suppressor use on a GI system!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:08
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...and I am outta here.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:11
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Kind of a solution to a problem that doesn't exist.

First, I haven't used the Adams uppers, so this is more piston vs DI in general.

With some time behind a SCAR-L or 2, the recoil of a piston gun is more jolting than a DI gun.  I was interested in testing the Rock River piston gun, but it wasn't ready for suppress fire when I had time to test.

On suppressed shooting, the source of most of the grunge is the barrel, not the gas system.  having shot DI and piston suppressed, DI might get a little dirtier, but not much, both are dirty as hell.

My ARs are all DI guns.  Were I in the market for a full-auto AR, it would be a piston gun.  In that , cooler fire is a great thing, some loss of accuracy is OK,  and cleaner is better.  For a semi, mine are mostly suppressed, so DI or piston doesn't make much difference.

My strings were slower on a piston gun than a DI.

Lastly, if your piston gun dies, good luck finding parts.  With a DI gun, stuff is everywhere for repairs.

I like the idea of a piston system, but have yet to shoot one I like, and with suppressed fire being so damned dirty, I don't see a benefit from the clean and cool argument.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:13
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I have a 10" AR15, I wouldn't use it for anything beyond 100 yards.  If you are looking at .223s and AR platforms, longer is better for 600-800 yard ranges.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:16
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Originally posted by Edsel Edsel wrote:

Hm.

Won't be getting any answers for barrel length, the way things are looking...


18-20" and 178 - 200 gr with reasonably high BC (above .450 or so) is GTG, but I thought we already answered the barrel part.  You're not going to get a consensus answer, though; opinions vary.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:17
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Wow and you wonder why people leave before they can get a good answer.          
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:18
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... bullet weights given your Sig Sauer 716 PM reference, which tells me you're interested in 7.62 rather than 5.56.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:19
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Originally posted by Edsel Edsel wrote:

...and I am outta here.

 


Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: October/31/2012 at 16:21
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Originally posted by Edsel Edsel wrote:

...and I am outta here.


Why?  You got answers to all your questions, and during the time we got off-topic, we were posting simultaneously before we had a chance to read your follow-up comments.

For my part, I apologize for helping steer the thread off-topic, however you did ask about barrel length considerations AND rifle design, and gas system length and type is directly related to both.
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