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Old March 6, 2013, 04:12 PM   #7
Join Date: March 4, 2005
Location: Ohio
Posts: 13,656

Actually, groups normally grow somewhat more with each additional 100 yards. as Bart was explaining. This is because aerodynamic drag is constantly slowing the bullet down. That means it takes a longer time to traverse each successive 100 yards than it did the previous 100 yards. The in-flight influences that open groups up are thus given more time to act on the bullet over each successive 100 yards. This includes wind, gravity, and any drift components to the flight, such as may be introduced by muzzle crown or bullet base imperfections, rifle disturbance during the bullet's barrel time, etc.

I don't know what length your rifle barrel is, but just to make an example with your bullet, QuickLOAD suggests that with a 24" barrel you'd get somewhere in the vicinity of 2200 feet per second from that load. QuickTARGET Unlimited, an associated exterior ballistics program, shows flight time of 0.1421 seconds to the target at 100 yards and 0.2968 seconds to 200 yards. That means it took 0.1547 seconds to cover the second 100 yards, and that's 9% longer for drift effects and other influences to act over the second 100 yards than over the first, and the effects of both must be added to one another for the total effect at the 200 yard target.

If the velocity estimate is close to what you actually have, I get 7" of drop between the 100 yard target and a 200 yard target. An extra 8 mph of 9:00 wind would move you 3" right at 200 yards, but I think you'd have noticed and remarked on that. If your barrel has any contact with the stock and you shifted your hold or its position on the bags any, that could account for both the missing 4" of drop and the rightward shift. So could a loose scope base or ring or other scope problem. For example, a scope with parallax set for 100 yards (common) and not adjustable for 200 could help throw you off. All speculation on my part, but about as much as I can suggest from behind a desk.
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