you do need a felt to be functional. as far as flopping open... the head of the piston is adjustable. look up how to adjust it... many videos made on it. dont get greedy... 395 fps is good (you'll understand the comment after you fix your gun ;)lolPiston adjustment made absolutely no difference. Not knowing what the problem is I have a new cocking lever assembly on order as well as a complete reseal kit.
I had one.I had a 777 too.There is a rivet in the cocking linkage I think that has loosened up.Maybe smack it with a hammer.