What size is the buffer retainer spring? Need to go get one at the hardware store to finish my build. Thanks!

Take the detent pin with you, fit spring into detent, spring should be about an inch long.

I probably have one I could mail you

I do appreciate that offer. If I can't find one at the store then I'll message you. Thanks!

Good advice, thanks

Remember the rule, 2 = 1 and 3 = 2.  If one spring stopped your build, another spring can stop your gun.  I keep a full spare parts kit and a couple kits of spare pins and springs.  Doesn't help your current build situation but I raise the idea for future benefit and for other readers of the thread who can get ahead of the flying spring mishap.

By the way - I'd heard this idea before but it didn't take until I lost my my own buffer retainer spring somewhere in the shag carpet on the other side of the living room and had to postpone my own build for nearly 2 weeks (never believe Midway's promise of fast shipping).  I eventually even found the lost spring weeks later but, in any case, I'll never be stopped for lack of a 10 cent part.

Unconfirmed:

Length: 0.940
Diameter: 0.180
# of coils: 14