I was gonna use a lead bb held in place by a dab of grease but then decided to use a 22 LR bullet super glued to the buffer. I decided on that because bullets would be uniform starting size and lead composition. And this whole experiment could be easily duplicated by another person wanting to add to the results.
I use the term buffer loosely as an M1A1 buffer with it's solid rivets doesn't provide any buffer cushion what so ever. I inserted one round directly in the chamber with no magazine in place. The goal was to get the straight recoil force of one round. It worked beautifully, the lead had a nice squish. I measured it and will retain it for the comparison to the other guns. I plan to test a 1921 bolt assembly and a 1928 bolt assembly and a semi auto 1927. In the 21 and 28 I will defeat the existing buffer cushion and have straight metal contact for the squish.