Will the Colt M16 bolt/carriers work in my semi AR or do they need to be modified to function correctly?

Yes, unless it has one of Colt's protruding sear blocks then you'll have to mill the auto sear trip face located underneath the carrier back slightly.

Edited to add, you can ignore all those folks that will now descend from on high and tell you that putting an M16 carrier into your AR15 is illegal.

Not only will it work, it will work better than an AR15 carrier. [;)]

Didn't someone get a letter from the ATF saying it was ok as long as you didn't have all the fire control parts and it could not be made to fire more than one round per trigger pull?

Thanks Tweak.I have a PWA lower so no sear block.I was actually looking at it because I haven't seen too many full-circle Colt semi carriers.

It is not illegal per say, but my question is "Why?".  They cost money you don't have to spend or that can go to a RIS or something.  The gun is still going to go bang once per pull of the trigger. Lastly, about legality...When it comes to the ATF you are dealing with a couple decent people (some of the tech branch guys) in a pool of government sharks with chips on their shoulders who's greatest thrill in life is exercising their power.  If that means visiting you in your house because you purchased an M16 part and "ooops" dropping an inch and a half long unregistered auto-sear in your sock drawer, you are screwed.  No lawyer in the country will touch you and all the Govt. lawyers will be on top of you.  I worked for a company that sold M16 carriers and we tried our best NOT to sell them to people without NFA weapons simply to cover our own asses and protect our customers.

1st of all I'm buying it new for less than a BM assembly and 2nd, I don't want a 1/2 circle Colt carrier.The M16 carrier is full circle.

DevL - why?  Are you still waiting on the KAC rather than spewing out the cash for the LMT?  LMT has semi & full versions, right?

I will have use of an LMT bolt and carrier (full auto) for a while.  Yes they have semi versions available too.  Someone was kind enough to loan me one for evaluation.  If I like it enough I may buy one but yes I still want to wait for the KAC unit to be released as my Bushmaster M16 carrier is perfectly fine for the moment.

Who is LMT?

Lewis Machine and Tool

http://ar15.com/forums/topic.html?b=1&f=6&t=147440