I'm having a hard time understanding something and I think you guys can straighten me out.  As I've said before, I'm trading for a Colt upper (large pin, post ban).  I'm going to build or purchase a lower for it (small pin).  I know that I'll need a pin that Bushmaster sells to mate the large pin to the small pin, that's no problem.  First what is a sear block and where is it located, i.e. upper or lower?  How will a sear block effect the type of trigger I can use?  I've heard guys talk about RRA two stage not being compatible with "sear blocks" but Jewell triggers are.  I'm willing to spend the extra money to get a good trigger that is compatible with any upper.  Thanks and I hope I didn't ramble too much.  Thanks,
Paul  

The sear block is directly above the safety, on the lower.  RRA 2 stage is not compatible, it's too long and the sear block won't let it in.  I actually got it all the way in but it still didn't work.  The trigger you need for a lower with sear block has to be shorter.

Soooo......then the only thing that I need to make the large pin Colt upper compatible with the small pin RRA lower is the adapter for the pin.  I can use any trigger in any small pin lower and it'll work with any upper, Colt or otherwise, correct?  The problem then, is installing aftermarket triggers in Colt lowers.  Okay, I've got it. Thanks,
L'man [50]  

Correct.  Its the lower and Colt lower specifically.  The other problem is if you ever want to put in a DIAS.  Wont go in a pinned Colt lower nor any lower that failed to machine the rear of the reciever to spec (some leave material that was supposed to be milled away in an M16).

Hey DevL, what's a DIAS?
L'man

It makes your semi auto AR-15 into a class 3 automatic weapon.

Drop In Auto Sear [peep]