Effects of reciprocating mass on accuracy? AR specific

[Tim K]

I have a new upper from JP. Another rifle I own from them gives excellent accuracy. I ordered this upper with their newish adjustable mass BCG and an H2 equivalent SCS. The goal was ultimate reliability. All total, the reciprocating mass of the new rifle is 6 oz heavier than that of my other JP.

I've spent a ton of time trying to work up a load for this new upper, and I just can't break 1 minute. Shooting the same Ammo in both guns, the new upper consistency gives groups about twice the size of the old rifle.

Out of frustration, I swapped the heavy BCG and SCS for the low mass pieces from the other gun. It was a very brief test, not at all conclusive, but it appeared that accuracy was going to be much better with the low mass components.

It seems counterintuitive to me. Heavy stuff ought to delay unlocking a few milliseconds, getting the bullet farther past the gas port before things start moving, if they do actually start moving before the bullet exits the barrel. The barrel is 14.5", mid length gas.

Anyone got a plausible explanation for why reducing reciprocating mass should improve accuracy?