Range Report Velocity problem

rck

Private
Full Member
Minuteman
Oct 9, 2011
38
1
Colorado Springs, CO
I have run into a strange problem I hope somebody might have some insight into.

When I chrono my load I am getting 2505 fps, but my come ups point to 2750 when I use the truing function in Shooter. Now after using Shooter a lot with a number of loads and guns I trust it immensely. The chrono? Not so much so, but it is close enough when it is working.

2750 seems awfully fast for 41.5 grs of IMR 4895 under a 168 SMK in a 16" .308 LMT MWS, but the physics are the physics so?

Any and all thoughts greatly appreciated!
 
Have you checked the tracking of your scope? Trusting the bullet is one thing but trusting the bullet using a scope that isn't tracking correctly will not reverse engineer a correct velocity.
 
Have you checked the tracking of your scope? Trusting the bullet is one thing but trusting the bullet using a scope that isn't tracking correctly will not reverse engineer a correct velocity.

The scope is good. I have not done a formal calibration on it but when I dial in a change it delivers it. Nightforce F1 in a Larue mount by the way. Zero has ben checked as well.
 
What kind of atmospheric data are you inputting? 2550 fps seams reasonable for a 16" gas-gun, 2750 sounds pretty optimistic. I would check you atmospheric data again, I am getting 8200ft Density Altitude right now at my house in Colorado. Station Pressure is 23.413 in of mercury COS (absolute pressure). I would run your numbers again, you can verify them with these links and run them through JBM & Shooter to validate your dope.

NWS Denver-Boulder, CO
Density Altitude Calculator
JBM - Calculations - Trajectory
 
The atmospheric data the other day, per my Kestrel, was 77F, 25% RH, 24.6 "Hg station pressure. For what it is worth the range used was Pueblo West, CO which is at about 5100 ft. I have Shooter set to use absolute pressure.

PS Shooter gives me good results with another rifle.
 
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I decided a couple years ago when I forced my chrono into retirement that unless your pushing it right to the edge of the supersonic range: Life is much simpler without a digital read out of velocity.
It definitely sounds fast but if your inputs are solid and verified and your drops are reliable and repeatable and therefor can be modified reliably for environmental conditions from that base line........what is the worry.
Scope works,check.
Rifle works, check.
Ammo works, check.
You know exactly what numbers to run in order to determine appropriate firing solution, check...............Sleep well.
 
I decided a couple years ago when I forced my chrono into retirement that unless your pushing it right to the edge of the supersonic range: Life is much simpler without a digital read out of velocity.
It definitely sounds fast but if your inputs are solid and verified and your drops are reliable and repeatable and therefor can be modified reliably for environmental conditions from that base line........what is the worry.
Scope works,check.
Rifle works, check.
Ammo works, check.
You know exactly what numbers to run in order to determine appropriate firing solution, check...............Sleep well.

There you go being all practical! How can I obsess over numbers on my iphone if I just go shoot and collect my dope? ;-)
 
OK had some more time this evening. Using the links kindly provided by delfuego I get a density altitude of 7777. JBM gives the same results as Shooter.

So next time I can get out to shoot I am going to reshoot everything and confirm my comeups. If Shooter wants 2750 so be it. I will have the hottest shooting LMT in town!

Thanks for your help guys!
 
rck,

The problem stems from your input into your ballistic program. The chrono number seems reasonable for a 16" 308. There is no way you are anywhere close to 2,700ft/s let alone 2,750ft/s.

inputting the proper density altitude is the way to do it. Density altitude is pressure altitude corrected for non-standard temperature.

Here is how you should estimate your density altitude;

1. Standard atmospheric altitude is 29.92" Hg (mercury).
2. Pressure drops by 1" Hg per 1,000ft (1"Hg/1,000ft) increase in altitude.
3. To obtain your pressure altitude, divide your sea level elevation (5,100ft) by the above.
4. Therefore, At 5,100ft, your pressure altitude is (5,100ft/1,000ft) x 1"Hg = 24.82"Hg.
5. Last step is your temperature correction factor; sea level is 59F, and temp varies by 3.5degF per 1,000ft of altitude.
6. your standard temp should be 59F-(3.5F*5.1) = 41.15F.

IFF the conditions at the day you chronoed your load was not 24.82"Hg and 41.15F, you have non-standard conditions and need to correct for them.

If your temperature was hotter, your bullet would encounter less air resistance and act as though it was shooting with a higher muzzle velocity. If the temperature was colder, then your bullet would encounter more air resistance however, the air resistance while higher than on a hotter day would still be significantly better than at sea level.

I don't know if you are able to input your conditions in the program, what i would do is enter the pressure i calculated above (if you did not record your pressure reading on the day of your shooting) and the temperature on the day you took the readings (or the temperature above if you did not record that info also).

Those inputs would "true" your ballistics to more closely match what you will experience in real life.
 
rck,

The problem stems from your input into your ballistic program. The chrono number seems reasonable for a 16" 308. There is no way you are anywhere close to 2,700ft/s let alone 2,750ft/s.

inputting the proper density altitude is the way to do it. Density altitude is pressure altitude corrected for non-standard temperature.

Here is how you should estimate your density altitude;

1. Standard atmospheric altitude is 29.92" Hg (mercury).
2. Pressure drops by 1" Hg per 1,000ft (1"Hg/1,000ft) increase in altitude.
3. To obtain your pressure altitude, divide your sea level elevation (5,100ft) by the above.
4. Therefore, At 5,100ft, your pressure altitude is (5,100ft/1,000ft) x 1"Hg = 24.82"Hg.
5. Last step is your temperature correction factor; sea level is 59F, and temp varies by 3.5degF per 1,000ft of altitude.
6. your standard temp should be 59F-(3.5F*5.1) = 41.15F.

IFF the conditions at the day you chronoed your load was not 24.82"Hg and 41.15F, you have non-standard conditions and need to correct for them.

If your temperature was hotter, your bullet would encounter less air resistance and act as though it was shooting with a higher muzzle velocity. If the temperature was colder, then your bullet would encounter more air resistance however, the air resistance while higher than on a hotter day would still be significantly better than at sea level.

I don't know if you are able to input your conditions in the program, what i would do is enter the pressure i calculated above (if you did not record your pressure reading on the day of your shooting) and the temperature on the day you took the readings (or the temperature above if you did not record that info also).

Those inputs would "true" your ballistics to more closely match what you will experience in real life.


As I noted in one of the posts above, I inputed the current conditions into Shooter as measured by my Kestrel. I also use station pressure. The measured pressure was 24.6. That is a tinge lower than the calculated number you give, but I was just south of a thunder storm too! So I think we are on the same page there.

Re temperature, it is my understanding (could be wrong) that Shooter does do the temperature correction, but that is an interesting point to look at. I ran the numbers again to compare the results at 41F as well as 77F. At 41F Shooter called for 0.1 mil more elevation than at 77F. So a smidge and I have noticed as big a difference in just how the iPhone rounds the numbers vs the iPad! I did the calculation for both 2500 and 2750 fps with the same result.

2500 FPS, 425yds, 41F, 2.7 mil required

2500 FPS, 425yds, 77F, 2.6 mil required

2750 FPS, 425yds, 41F, 2.2 mil required

2750 FPS, 425yds, 77F, 2.1 mil required

I fiddled with humidity but that is a big so what.

Now at this stage, I have to wonder about the ammo too, but I have shot this load for over 30 years and it has been stable as a rock. I double checked my log and the only difference was a new scale that reads 2-3 tenths lower. I throw with a Harrel though and the number of Harrel clicks is a constant since I choose to control volume rather than mass. All that said, the numbers reported by the chrono past the smell test.

One last bit to toss in. This is a fast barrel and it chronos 50fps faster than my 20 Remington.
 
Couple more thoughts.

Looking back for mechanical variables a big one is shooting a .308 AR. That took some time to digest. The second is shooting off a bipod. That is relatively new to me as well these past couple of years. Can a bipod repeatably shift matters a half mil?
 
rck,

re-read your posts...did not realize you had your own atmospheric observation station! You are going about it the correct way. But at least now you know how to work out your conditions by hand if your krestel craps out or you forget it at home :)

What kind of shifts are you referring to with a bipod? I find bags to be much more stable platforms than bipods when shooting ARs...bipods dont cause an increase in POI for me, but they do cause significantly worse groups and inconsistent shooting compared to rests/bags.

Lastly, consider the powder you are using...IMR-4895...great powder however, not particularly temperature stable (compared to the Hodgdon extreme series). I would expect an uptick in velocity in hotter months, and a reduction in velocity in colder months.
 
Tx_Flyboy,

Never hurts to be able to do it by hand that is for sure!

Re bipods, it is not that I am seeing something, but I am wondering if there is a replicable effect. I have always been a bag guy in the past, but decided to learn to shoot a bipod these past couple of years. I am not seeing anything per se, but it took a fair bit of work to get decent with the bipod and I am still not 100% with it.

IMR-4895? I must be the odd guy out, but I have had much better results with it over the years than with the extreme series powders. Of course I am not pushing it to the wall either. Varget and H-4895 just have never clicked for me. The powder that scared me to death both times I tried it is 748, not one I want to standardize on living in Colorado with the wide temp swings possible from winter to summer. BTW, my loading philosophy is find something that works and then make it by the metric sh*t ton.
 
OK. Went to the range after work. Howling wind (25-30 & gusting) so results purely qualitative, but the 2725 fps dope looks pretty good. Shot for shot hits despite being blown all over the place so.... Anyway, some time when it is calm and I am free I will shoot it for groups, but right now? I am going to run with it until our 600 yd range is reopened or I get somewhere where I can shoot farther than 425.