The "C" clips are on the inside of the receiver. I was having problems with the hammer pin sliding out on one side and making the trigger useless. The pin had no grooves in it - manufacturer oops? This Bushmaster is not your typical AR. This is not the firearm that I am shopping for a new trigger group.
Like i said, I don't have any experience with drop in (aftermarket) triggers. I am just now shopping around for a replacement trigger set.
Ahh that makes sense, I'd be a little nervous about having those c-clips come loose and bind something up.
Word of caution, once you start putting in aftermarket triggers (drop in, cassette whatever) you'll get pretty spoiled and not want to go back to shooting mil-spec!
No big deal if you only have a couple ARs, could be expensive if you have many...