Peter
The ACR L1A1's are rechambered in .308, and hard extract with 7.62 ammo, so we can't use that.
The NATO spec 7.62x51 ammo appears to be in the 147 to 150 grain bullet range. The PRVI is 175 grains. I plan to step back to their 145 grain load when I buy the next batch (I've still got 190 rounds of 175 to use........:o ).
I take your point re the gas recoil mechanism. I said you had forgotten more than I know on these rifles :D
I must put mine on the scales to see what it weighs, compared to the original spec.
If you weren't quite so far away i'd let you shoot mine to see what you think.......:)