First competition and ballistics question

[Jasper1573]

Hello all,

I competed in my first 500 yard F-class competition this weekend. I shot in both the F-Open and F-T/R. I placed 5th in the Open and 1st in the TR, so I was well pleased and surprised as well.

Everyone there (Brock's Gap Training Center in Hoover, AL) was incredibly friendly and made me feel very welcome. My wife spotted for me, and we both had a great time!

Now the ballistics question...because I had my gun zeroed at 300 yards and had never shot at 500 yards before, I had to trust in an on-line ballistics calculator for the 500 yard scope adjustments; the ballistics calculator seemed accurate and agreed with the ballistics table in my Speer manual, so I am trying to figure out why the calculator called for an additional 28 clicks up in elevation (7 MOA) but in the competition, I had to use 31 clicks up (7.75 MOA). I don't think it was the temperature or elevation as both were very close to the same temp and elev when I zeroed the rifle at 300 yards. I also discounted my reloads as I have shot a couple hundred of this particular load and they regularly clock in at an average 2625 fps.

So, any ideas and or advice on the difference in calculated ballistics and actual?

Cheers,

Jasper

 

[Jim Watson]

The little bit of F class I have shot, I always felt either good or lucky to get within 3/4 MOA of zero with calculated "come-ups."

The difference is probably because your bullets did not actually have as high a Ballistic Coefficient as cataloged.
Can you back calculate to get a tested value of B.C.?

 

[Jasper1573]

Jim,

I used the same ballistic calculator and dropped the BC down from .496 to .400, and got 31 up clicks of elevation adjustment. That is really a lot, and the bullets are 175 grain HPBT Sierra Matchkings...I can't imagine that the BC of a Sierra is off by 20%...that seems like alot for such a high quality bullet. I will keep this in mind and see if this works out for other ranges and loads.

Jasper

 

[Howard Roark]

Sounds like the old G1 and G7 BC switcheroo. Berger now lists the G7 BC as well as the G1 BC for their bullets.

The algorithim that the BC program uses affects the programs accuracy as well. There are two methods, closed form and time stepping.

Check out Brian Litz's AppliedBallistics.com for the very best info there is on exterior ballistics for competiton shooters.

We have monthly 600 and 1K yard F-Class matches at Ft. Benning Ga. If you're interested please see our web site: www.crcrifle.com

 

[Jim Watson]

Been thinking. A bad habit, I know.

Also consider the possibility that your scope adjustments are not quite 1/4 MOA.
If your click value was .226 MOA instead of .250 MOA, it would take 31 clicks to move 7 MOA instead of 28. This would be very hard to measure other than actual shooting at fairly long range.

 

[bhw4235]

I agree with the scope adj. not quite being 1/4". I have run into that alot, even with higher end scopes.

 

[fractal7]

If your scope is not precisely vertical in relation to the bore as well, each step up might be more like 0.2 MOA up and 0.05 MOA to the right. It would be hard with windage to notice the slight offset but that could also be why clicks are sometimes "off" on the scope.