Stupid Moves

[Pitt300]

Today I was @ range w/AR-300-221 & Colt H-Bar 5.56.
I reload my 300-221 ammo into old Win USA 5.56 boxes & put an Avery label on them w/load data.
Today I got distracted & put a 300-221 in my Colt H-bar 5.56 & pulled the trigger.
LUCKY for me nothing happened!
Was I just lucky or will the 300-221 go in the chamber too deep for the firing pin to hit?
I'm sure I would have blown up the 5.56 had the 300-221 round gone off!