I would start with a good copper remover like wipeout. You can leave it in overnight. While it is soaking, I would examine your brass.

100 pieces of brass and you BELIEVE the round count is 375. You don't know how many times each was reloaded. Hmmm, let's see. Since you believe it may be 375 shots, let's call it 400.
400/100 = 4 reloads/brass

But since you did not separate the brass, that means it could be north of 5 or 6 or 7 reloads. It may be time to anneal or try a box of new brass and see what gives.

Don't forget you are dealing with a hotrod.


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dogzapper

After the game is over, the king and the pawn go into the same box.
Italian Proverb