We use to up the RPMs by putting quarters in the buffer tube to shorten the recoil stroke. I don’t know origins of it but we had 1 BAR. It stayed in 4/60 bunker on FB Blackhawk. The only other BARs I saw were in civilian contractors office in Chu Lai. They were hosting MARS calls for married men and was there with buddy. His call went pretty fast, didn’t give me time to figure out how to boost a BAR.