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Posted: 7/25/2017 4:21:53 PM EDT
This is my first post to this board.  Are all Colt AR15 uppers compatible with all Colt AR15 lowers?  If not, why not.  Thanks.
Link Posted: 7/25/2017 9:57:42 PM EDT
[#1]
No. You have some machinegun uppers that are open bolt that will not work on lowers. Early colt civilian uppers had a large hole in the front and it takes an adapter to put a standard upper on one of them. You do have adapters.

Also on some colt lowers they have a auto sear block that prevents a full auto bolt carrier from working with them. You could switch to a semi auto carrier and be fine though.

Edit: While not asked some of those older colt lowers had larger fire control pins in them. Means you need special parts to replace the trigger group(or bore out standard ones).
Link Posted: 7/26/2017 1:25:09 AM EDT
[#2]
Thanks for the info.  I bought a Colt 7.62 x 39 upper several years ago, and the barrel was manufactured in the 1990 -- 1995 period.  I was thinking of buying a new Colt M4A4 lower, but did not know if there would be a compatibility problem between the upper and lower.
Link Posted: 7/26/2017 1:27:47 AM EDT
[#3]
Discussion ForumsJump to Quoted PostQuote History
Quoted:
Thanks for the info.  I bought a Colt 7.62 x 39 upper several years ago, and the barrel was manufactured in the 1990 -- 1995 period.  I was thinking of buying a new Colt M4A4 lower, but did not know if there would be a compatibility problem between the upper and lower.
View Quote
The newer colt lowers are pretty much in spec with everyone else. Standard trigger pins and they don't have a sear block that sticks out the top. It should take pretty much any standard upper(other than the large front hole uppers without adapter).
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