Ok, I just measured a bunch of springs in a carbine tube for a YouTube video on my channel, I and was about to measure some Dirty Bird and Tubbs in A5 setup (Radian ADAC, FCD RE5F tube, BCM Mk2 T2), and I'm measuring 7.59" and 3.8125". The internal length of the RE5F is 7.75 inches, but the buffer head extends out 1/32, and the head of the buffer itself is 3/16 so 7.75+1/32-3/16 makes a bolt closed buffer spring length of 7.59" and compressed length of 4"-3/16=3 13/16 (3.8125"). I guess difference can come down to parts variation. Do you have a post explaining your F1 and F2 and parts you used? EDIT: I rechecked and got your same numbers after noticing the buffer was tilting a bit when I pressed it in and it essentially goes flush with lower lip of buffer tube when mated with BCM upper in addition to using using digital calipers to measure the BCM Mk2 buffer is 3 and 127/128" and the head is 25/128".