Thanks for your help! Turns out, my gun's not broken, it's just automatic. It all becomes clear when I take it to the range, load it, put one in the chamber, and pull the trigger. As if by magic, the 121 m1 loads itself 7 more times! Ya'll should have told me to quit playing with it and go shoot it.
I have same problem as Zardoz. Did anyone ever answer? I have same gun, same wasted 2 hours, same little button, same result. Is my 121 M1 broken as well?