the manual suggests using your index finger, and the associated instructional figure shows an index finger pressing the shell retaining lever. but i've tried thumb, middle finger, multiple fingers, etc., and it's almost impossible to unload shells from my M4. i can usually get a couple shells to unload, but by then i'm sweating, spitting, cussing, swearing, etc. and i finally give up and just unload the remaining shells by cycling them out. i know everybody struggles a bit with this issue, but mine seems 100 times worse. i installed a GG&G tactical bolt release pad (i don't think that would matter). as i recall i noticed the same difficulties unloading shells from the minute i first got the gun.