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My 308 is doing 2835 FPS? Help

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 13:02
Gil P. View Drop Down
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I did a test at 300, 500, and 700 yards to check the drop of my bullet and determine its velocity using a ballistic calculator (jbm). At 315 yards my drop was 11.5 inches, at 500 yards my total drop was 49.6 inches. The 700 yard bullets dropped too low to get on paper and I could not recover any data from them.

I fired three shots at each yardage and took my data from the middle of each group.

Upon plugging in my data to JBM I determined a velocity of 2835 FPS, which seems VERY fast. Its hard to argue with my data, but I wonder if JBM is inflating my velocity.

I am shooting a 308 out of a Rem 700 with a 24inch Bartlein 1 in 11.25 twist barrel.

My loading data is: Lapua brass, WLR primer, 44.6gr of VV N150, 175 SMK bullet.

I measured the distances using a Leica LRF 1200 rangefinder.

I live at about 2500ft above sea level.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 13:19
BeltFed View Drop Down
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I'm not the expert on or with ballistic calculators, but what was your zero when you fired your groups? If you have a 100yd or 200yd zero, it will make a difference on how much drop you will have at 3,5,and 7 hundred yards. I'm guessing that you made a mistake somewhere, because unless your showing signs of high pressure, 2,835fps seems very high for a safe load for a 175gr bullet and 24" barrel.

Do you have access to a chronograph?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 17:12
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I wouldn't rely on a computer program to figure bullet drops, MV, or anything else.  Gotta use a chronograph to know for sure.  
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 19:09
jonoMT View Drop Down
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Ballistics calcs are very dependent on correct input. Sounds like something's not quite correct. Can you post the values you entered into JBM?
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 20:57
Mike McDonald View Drop Down
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4.25 moa at 300
10.50 moa at 500

That's with 175's at 2625 at 1100 ASL on an 85 degree day and 28.85Hg station pressure.

Sounds like you're right in that ball park.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 23:17
Gil P. View Drop Down
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Here is the data I inputted. My own measured data shows that the bullet drop from 100 to 300 yards was 11.6 inches. The drop from 300 to 500 yards was 38.1 inches for a total drop from 100 to 500 yards of 49.7 inches.

The drop in inches is the second column and the drop in mils is the third column.

The data for the weather was taken from Boulder City, NV

Trajectory
Input Data
Manufacturer: Sierra Description: MatchKing™ (Litz)
Caliber: 0.308 in Weight: 175.0 gr
Ballistic Coefficient: 0.243 G7 (ICAO)    

Muzzle Velocity: 2835.0 ft/s Distance to Chronograph: 10.0 ft

Sight Height: 1.50 in Sight Offset: 0.00 in
Zero Height: 0.00 in Zero Offset: 0.00 in
Windage: 0.000 MOA Elevation: 0.000 MOA
Line Of Sight Angle: 0.0 deg Cant Angle: 0.0 deg

Wind Speed: 10.0 mph Wind Angle: 90.0 deg
Target Speed: 0.0 mph Target Angle: 90.0 deg
Target Height: 12.0 in   

Temperature: 90.0 °F Pressure: 29.91 in Hg
Humidity: 8 % Altitude: 2410.0 ft

Vital Zone Radius: 5.0 in   

Std. Atmosphere at Altitude: No Pressure is Corrected: Yes
Zero at Max. Point Blank Range: No Target Relative Drops: Yes
Mark Sound Barrier Crossing: No Include Extra Rows: No
Column 1 Units: 1.00 in Column 2 Units: 1.00 mil
Round Output to Whole Numbers: No   
Output Data
Elevation: 3.785 MOA Windage: 0.000 MOA

Atmospheric Density: 0.06597 lb/ft³ Speed of Sound: 1149.3 ft/s

Maximum PBR: 349 yd Maximum PBR Zero: 296 yd
Range of Maximum Height: 163 yd Energy at Maximum PBR: 2014.7 ft•lbs

Sectional Density: 0.264 lb/in²   

Calculated Table

Range

Drop

Drop

Windage

Windage

Velocity

Mach

Energy

Time

Lead

Lead


(yd)

(in)

(mil)

(in)

(mil)

(ft/s)

(none)

(ft•lbs)

(s)

(in)

(mil)
drop (in) drop (mil)
0 -1.5 *** 0.0 *** 2840.7 2.472 3135.1 0.000 0.0 ***
100 0.2 0.1 0.6 0.2 2672.2 2.325 2774.3 0.109 0.0 0.0
200 -2.9 -0.4 2.4 0.3 2509.2 2.183 2446.2 0.225 0.0 0.0
300 -11.6 -1.1 5.5 0.5 2352.0 2.046 2149.3 0.348 0.0 0.0
400 -26.6 -1.8 10.1 0.7 2200.8 1.915 1881.8 0.480 0.0 0.0
500 -48.7 -2.7 16.4 0.9 2055.4 1.788 1641.4 0.621 0.0 0.0
600 -79.1 -3.7 24.4 1.1 1915.2 1.666 1425.0 0.772 0.0 0.0
700 -119.0 -4.7 34.4 1.4 1779.5 1.548 1230.3 0.935 0.0 0.0
800 -169.9 -5.9 46.7 1.6 1648.2 1.434 1055.4 1.110 0.0 0.0
900 -233.7 -7.2 61.4 1.9 1521.2 1.324 899.0 1.300 0.0 0.0
1000 -312.5 -8.7 79.1 2.2 1398.8 1.217 760.2 1.505 0.0 0.0
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/12/2013 at 23:24
Gil P. View Drop Down
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Originally posted by Mike McDonald Mike McDonald wrote:

4.25 moa at 300
10.50 moa at 500

That's with 175's at 2625 at 1100 ASL on an 85 degree day and 28.85Hg station pressure.

Sounds like you're right in that ball park.


Mike, JBM says my drop is 3.7 MOA at 300 and 9.3 MOA at 500. It was a 90 degree day and 29.91 station pressure.

I know something isn't right though. The distances shot were correct, and shot level to one another. I wasn't shooting uphill or downhill. Could JBM be wrong? Would the drop tables it gives me still be useable?

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 02:52
Gil P. View Drop Down
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Originally posted by BeltFed BeltFed wrote:

I'm not the expert on or with ballistic calculators, but what was your zero when you fired your groups? If you have a 100yd or 200yd zero, it will make a difference on how much drop you will have at 3,5,and 7 hundred yards. I'm guessing that you made a mistake somewhere, because unless your showing signs of high pressure, 2,835fps seems very high for a safe load for a 175gr bullet and 24" barrel.

Do you have access to a chronograph?


I do not have access to a chronograph, that is why I tested my drop this way.

My zero when I fired my 100 yard zero was between 100 and 115 yards. I was on a bit of a hill shooting down. The other shots taken at the test target that I retrieved my data from were all taken almost level.

I agree it doesn't seem to make sense. Maybe im just lucky and there is some sort of black magic at work here. Or maybe I should try a few different ballistic calculators, although I hear JBM is highly regarded.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 05:35
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I used to shoot 155gr bulltes at 2850 and I got the same drop table as you are getting.
 
So to me your speed calculation seems correct.
 
My speeds were over a chrony and the drop table from JBM. This correlated correctly with my actual drop measured on the range.
 
I still prefer to shoot at actual distances eg 250 yds, 300 yds etc and then plot that actual drop into JBM and calculate speed from there. This gives me actual drop, which is more important then actual speed. Chronies are not all that accurate.
 
When I do measure over a chrony at a later stage I am in the ballpark as crunched out by JBM. So the backwards calculation works.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 07:18
jonoMT View Drop Down
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Your inputs look good and JBM is the gold standard of ballistics calculators. I know when I wrote mine for Windows Phone, that's what I tested against. So I wouldn't worry about it as a factor.

Sounds like you got yourself a super-fast barrel. If so, enjoy it to the hilt! I had a Rock Creek that would push 175s as high as 2795 on top of Varget...70 more fps than I ever got from a stock Rem. 24".
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 07:19
Mike McDonald View Drop Down
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Gil,
What altitude are you shooting at?
Asking because 29.91 station pressure is pretty high. Sure that isn't corrected sea level pressure?

Of course, with a tight barrel and maybe a palma 95 chamber?? you could just have a fast rifle.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 08:28
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It's all about pressure.  Can't get abnormally high velocity without increased pressure. 
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 10:18
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My 19" obermyer barrel will push around 2660 with 175s and 44.5 varget with Lapua brass.   It has a real tight chamber and barrel.  That is as fast as my 25 in Lilja barrel would do.   
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 14:35
Alan Robertson View Drop Down
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That would be an amazing speed from a single- base powder, but there are so many vagaries to this game, that your calculations might be correct.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 16:58
jonoMT View Drop Down
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Originally posted by Mike McDonald Mike McDonald wrote:

Gil,
What altitude are you shooting at?
Asking because 29.91 station pressure is pretty high. Sure that isn't corrected sea level pressure?
It looks like Gil checked the box to use corrected pressure and supplied the altitude.

FYI, for those new to ballistics calculators. You can deal with atmospheric pressure in a couple ways (and there are others, such as Density Altitude, which boils all the factors affecting air density into one number):

  1. If using a corrected barometric pressure (typically what you get from weather reports) you need to supply the altitude
  2. If pressure is uncorrected, i.e. "station pressure" then altitude needs to be left at zero or none

Since I usually take pressures off my GPS, I prefer to use the second approach because I can leave altitude alone.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/13/2013 at 17:58
Gil P. View Drop Down
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Thanks for your input guys. It looks like I may just have a fast barrel.

In case this makes a difference:

The Bartlein barrel I have on my rifle is a modified medium palma. I used the MP contour as starting point and then changed it slightly to suit my needs.

I don't suppose palma type barrels are made with tighter bores, but I thought I would post this anyway if it makes a difference.

The rifle was chambered by Brad at Phoenix Custom Rifles, they have great customer service by the way.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/18/2013 at 10:13
Gil P. View Drop Down
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Upadate

I was at the Score High Tactical Rifle Challenge in New Mexico over the weekend (which was a ton of fun) and noticed some things.

1) The dope for 2835 FPS was working perfectly the first day of the competition and also very well the second. The altitude of the nearest city Albuquerque was about 5100ft, and pressure was 29.95 compared to my 2500ft and pressure of 29.91.

2) I noticed my range finder was not adding up to the posted distances of the targets. For instance, I was getting 508yds at a target that was 550yds away. I had other people check the distance with their LRF and they were getting 550yds. After the competition I noticed that my LRF was set to meters, not yards. Which would indicate that my bullets are going even faster than I thought.

3) Someone had a chronograph with them, which they let me use. My average FPS was 2760FPS. And now I have a question: My barrel at the end of the competition was very dirty. I had around 150 rounds through it and the conditions at the competition were extremely windy. There was a lot of sand blowing around. So much so, that one competitor could not close his action all the way, even though we all kept them covered. How much velocity would be lost with a dirty barrel?

4) When I collected my data in my first post, I let the rounds heat up a little bit because I knew that would be a normal condition for them to be in and I wanted to be sure that they would be safe if they did heat up. This might have raised their velocity a little bit.
When we chronoed those rounds it was overcast, so I may have lost some of the velocity that I started collecting data with.

Ill be purchasing a chronograph in the future, but if anyone has any opinions on this, please let me know.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: June/18/2013 at 11:09
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  "When we chronoed those rounds it was overcast, so I may have lost some of the velocity that I started collecting data with."
 
  Actually,I prefer to use my chronograph in overcast conditions.  Offers kind of a "natural" diffuser that eliminates false readings due to shadows that can be a concern on sunny days.
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