Today I finally got to test out a recommended handload that's supposedly very accurate.  After having a hell of a time getting the cheap 'scope sighted in,  I nailed some really beautiful shots.  Got a few honest dead-center bullseyes at 100 yards.

But I had consistency problems.  As the barrel heated up, (Bushmaster National Match 20 inch heavy barrel on an AR-15 type rifle, configured entirely as an A2 except for an A1 upper receiver), shots started stringing DOWN on the target.

Maybe I was simply shooting too fast.

I did notice that after each target maintenance session, where the barrel had had about five minutes to cool down, the first shot was ALWAYS a bullseye...once the 'scope was sighted in, that is.

I DID discover that even a big, thick, heavy barrel like this one is not so stiff that you can't throw its aim off by resting it on the sandbag near the muzzle end of the handguards.
Doing that threw the point of aim up more than half an inch at 100 yards.  So if I must use the
sandbags to rest the handguards on, I push the rifle forward until the mag well is touching the sandbags, and that helped a lot.

So, would anyone care to enlighten me as to what rate of fire is generally considered to be best for maintaining best accuracy  with a barrel of this general type?

CJ