Further tests w/ a 1944 M44 and a '91/56 had the same results.
Also tried some factory 21(Hungarian) '76 and '85? vintage as well as some bxn from the '60's -no dramas.
Guessing the average loading force to be about 10-15 lbs!
Do it sort of like this except rotate the loading hand 180 degrees. (thumb rearward!)
http://www.kmike.com/KWjpg/SniperM1C-M82Sight.jpg