Range Report Shooting 0.75 MOA low @ 600yards going by software

[alpha6164]

This weekend i took my GAP 308 with 26" barrel to the 600yard range for some shooting. Living in Florida we are at sea level and temperature was 75degrees. I have previously chorono'd this rifle with Federal Gold 175 and i was getting numbers from 2710-2730ft/sec, so i used 2720ft/sec for calculations.

When i plug in the following:

BC=0.497
Velocity= 2720
Sight height= 1.7in
Sea level, 75deg, 90% humidity,

It says that i need 14.3MOA for 600yards. Dialing 14.25MOA i am shooting consistently 5-6" low. I had my buddy shoot with same settings and we get same result. If i dialed 15-15.25 MOA elevation then i am right on. The rifle itself performed flawlessly. My groupings were right about 3" and my buddy was a little better than 3".

So is this common where the ballistic software doesnt match up? The only way i can get 15MOA is if i either reduce the velocity to like 2680, or input a lower BC. Also, if i use G7 BC of 0.243 doesnt really make much difference either. I would like any inputs. Thank you.

 

[doc76251]

Re: Shooting 0.75 MOA low @ 600yards going by software

2720 fps is a bit high for FGMM 175's, 2680 is more in line w/ a 26" tube.

Does your data track low all the way out, and beyond?

Have you calibrated your scope so that you know you are getting true MOA from your dials or are you getting IPHY or some other value?

Ballistic programs (especially inside 500) are VERY accurate when provided good data.

Usually it's the velocity or the scope.

Cheers,

Doc

 

[alpha6164]

Re: Shooting 0.75 MOA low @ 600yards going by software

My scope is a NF 5.5-22x50 NP-R1. When the software asked for 14.25 MOA, i dialed exactly 14.25MOA on the elevation turret. 600yards is the farthest i have shot the rifle so far. There are really not that many places in North Florida that gives me more leg room. The shots were a little low at 300yards, but not enough to be that obvious. But at 600yards it was definitely obvious. Maybe my chrono was a little optimistic

 

[Rafael]

Re: Shooting 0.75 MOA low @ 600yards going by software

Continuing with what Doc said......

What was the air pressure?
The pressure changes all the time. "Sea level" pressure is no more consistent than the pressure at 5000 feet.

Chronos seldom agree. Test the speed with two different chronos and you will get two different speeds reported. You are having to offset the chrono speed by less than 2 percent to get your dope to agree. That is not a great deal of difference.

There are many variables that must be included to get an accurate result from a ballistics program. The one we can be least sure of is velocity. This is why most people use real-world dope to spot inconsistencies and "adjust" the velocity or other variables(like scope adjustment values) until the dope matches.......trying to be "more" accurate with each of these variables where possible. Test the scope to be sure it is not generating the error.

http://www.arcanamavens.com/LBSFiles/Shooting/Downloads/ScopeChecking/

Only after ruling out other variables can one get a fairly accurate prediction of results from calculations. Shit in, shit out.

 

[Sniper1*]

Re: Shooting 0.75 MOA low @ 600yards going by software

Running the numbers you have with out a known air pressure (I used 29.60) and the MV @ 2680 Ballistic FTE says 15.1 MOA up!

That is a pretty generic answer given what we have to work with but I would guess chrono error.

I would definitely do the scope tracking drill any way though!

Good Luck!

 

[Killswitch Engage]

Re: Shooting 0.75 MOA low @ 600yards going by software

you should use .475 for your bc. i find that numbers tracks much better in my program. fwiw i have 15moa on at 600 for 26" 1/12 with 175 smk @ 2650.

 

[Lowlight]

Re: Shooting 0.75 MOA low @ 600yards going by software

A ballistic solution based on software is a suggestion, nothing more and to be within 1 MOA is more than acceptable without tweaking the software to account for your rifles' unique way of shooting.

There are so many variables to account for, expecting a 1/4" minute solution is pretty unrealistic, especially 600 yards and beyond. It can happen, but usually with some adjustment to the program, even then, it can change from day to day.

One has to ask themselves how accurate is my chronograph, your error there is enough to cause a problem, not to mention .497 is being generous, then as others have said, environmental conditions, well you get the point.

Nothing beats actual data, and even then, expect it to change from day to day slightly, and then change more as you shoot, and then change more with different lots of ammunition, and then change again as you adjust to shooting that rifle.

Its a starting point, nothing more, after that you go with what you know which is based on the actual shot taken then, and no time else.

 

[mm509]

Re: Shooting 0.75 MOA low @ 600yards going by software

Somebody please correct me if I am wrong, but arent commercial listed ballistic coefficents directly affected by velocity of the said bullet? Or do these ballistics programs take in to account the drop in B.C.'s for the drop in velocities? I know Sierra for example lists at least 3 different B.C.'s that have a direct corealation to the velocity.

 

[flyboy]

Re: Shooting 0.75 MOA low @ 600yards going by software

Brian Litz has a free balistic calc online that he has his numbers witch take all that into acount. They have been spot on with all the loads I've tested.

 

[mm509]

Re: Shooting 0.75 MOA low @ 600yards going by software

flyboy do you have a link to Brian's calculator?

 

[goober]

Re: Shooting 0.75 MOA low @ 600yards going by software

I shot a 500 yard comp in the weekend and on the first shoot with a very kind wind put 9 of 10 into the bulls eye - one that went to be a 5 may have been me or some sneaky wind - anyway to get to the point I used 3.1 mills and 3.2 both got me there but I felt like 3.15 would have been perfect -Bryan Litzs berger caculator was bang on with that very number !!!