I cannot get aluminium checks to work with my .375 H&H. I've tried various thickness's of metal, .014" - .022" and get the same result; zero accuracy. My usual high velocity load of a 300 gr. round nose at 2200 fps give me groups under 3/4" at 50 metres. With aluminium gas checks I am lucky to get all the shots on to the 30" square backer.
I've isolated the gas checks as the problem using the same batch of bullets, sized, lubed and checks seated identical. But the results are the same; Hornady checked bullets go in to nice small groups, alu. checked bullets all over the place. Even if I shoot alternate rounds of copper and alu. checked bullets I get the same result. They are not grouping at a different point of impact as they are going in completely random directions.
The alu. checks snap on to the bullet base as they should and after sizing are quite tight and are the same diameter as the driving bands (.377"). Some of the checks do leave the bullet when the bullet exits the muzzle but have no sign of gas blow-by. I've examined the rifle bore with my Olympus bore-scope and there is no sign of lead fouling or of anything different from what I see when using copper checks.
So what's going on?
I also make checks from the same materials for my .30 caliber rifles and they shoot just as good as those with copper checks.