What do you think the muzzle velocity of the load that you are shooting is? Is it supersonic at the muzzle? If so, how far ABOVE supersonic do you think it is? Could the ball be supersonic at the muzzle and out to 50 yards, but then drop to subsonic speed before it gets to 75 yards?
I shoot NRA Highpower Rifle competition. When I was starting out a couple of years ago I was shooting an AR-15 with a 1:9 twist barrel, and it would only shoot 69 grain bullets accurately at 600 yards. It is desirable at that distance to shoot heavier bullets, and most competitors were using either a Hornady or Sierra bullet of around 80 grains (Heavier bullets are less likely to be pushed around by the wind.). I bought some Hornady 75-grain bullets that were supposed to be able to be stabilized in a 1:9 twist barrel.
Early in the season they seemed to shoot pretty good at 600. But, depending on the weather - temp and humidity - the accuracy went all to Hell. Not only that, when they hit the target, they were leaving a "keyhole" in the paper.
What was happening was this: The load I was using would launch the bullet at supersonic speed. On the 200 and 300 yard targets the bullets would still be travelling at supersonic speed. But sometimes, depending on the weather, the bullet would slow down to subsonic speed before it made it to the 600 yard target. When it did so, the standing shock wave that was being "pulled" along by the bullet would hit the base of the bullet when it slowed to subsonic, kicking the bullet sideways and affecting the trajectory. I know they were subsonic at that time because the guys in the pits pulling and scoring the targets couldn't hear the "crack" of the supersonic bullet passing over their heads.
To fix this I did three things: I switched to 80-grain Sierra Matchkings, and I started to load them to a C.O.L. that was longer than would fit in the magazine (which was OK, because for the 600 yard stage you are shooting slowfire and load each cartridge manually by hand directly into the chamber for each shot). By loading them long I had more room in the case for powder, so I could keep them supersonic past 600 yards. I also put a different barrel on my rifle that had a faster 1:7 twist, so it would shoot the heavier bullets. I shot the 69-grain Matchkings at 200 and 300 yards, and used the 80-grain Matchkings at 600 only.
I suggest that you try one of two things: Either increase your load to see if driving the ball faster keeps it supersonic for a greater distance, or alternately, load it down so it doesn't go supersonic in the first place.
As we say here in LA "Happy Happy Happy".
