Here's a pic of the two (quick and dirty pic with my phone, and yes, those are .45ACP rounds propping them up behind the gas tube):
Top is a semiauto RRA BCG. Bottom is a FA Spike's.
Note at the bottom rear (left side) of the carriers. The FA (bottom) has the cuts even at the top and bottom. This is so the bottom of the carrier can catch the sear in a FA lower. The semi (top) is cut further to the rear to prevent it from ever being used to trip an auto sear.