I have a Colt lower that still has the sear block in it.  I was told that a standard bolt carrier will not work unless the sear block is removed.  I placed my bushmaster upper onto my Colt lower and cycled the bolt a few times and never noticed any contact between the carrier and the sear block.  Also, there was no noticeable wear on the surface of either piece.  From what I can tell there is just enough clearance for it to work.  Was someone just pulling my leg when they told me this or is it true?  I have not attempted to fire it in this configuration yet because I wanted to find out from you guys if it can be done first.  Thanks.

The sear block Colt lower will only prevent installation of a drop in auto sear.  It will function with normal AR parts like you described.  The Colt "half-moon" bolt carrier normally associated with this lower was designed to prohibit use of the AR-15 to M-16 bolt carrier converter device.  The sear block which is easily and legally removed has no effect on any "full-circle" bolt carriers.

It should work fine.  Watch-Six

Thanks guys.  That's what I wanted to hear.  I didn't really want to remove it unless I had to.

My M4 colt upper with the M-16 carrier will not close onto my pre-ban colt lower with the sear block. Put the ar 15 carrier into the M4 upper and no prob. So check first. Just my experience.
Spooge Out!

Bingo!  We have a winner!  You just discovered why Colt put in a sear block with a piece of it protruding up into the upper receiver.  The block serves two purposes, you cannot use a sear or a M16 carrier.  If you ever looked at a Colt AR15 in 9MM with a sear block, you would notice that this part of the block is cut off because the 9MM carrier is the same for semi and full auto.  Go figure!

What did I win!?[bounce]  What did I win!?[bounce]

I'll take door number........ [thinking]

3! [party]