Thread: AR-15 from 9mm to 5.56???

Looking for some knowledge from AR builders out there. I have a SBR’d 5.56 AR that I bought already set up as a 9mm (mag block for Uzi mgs and complete upper) . I would like to put a 5.56 upper on it but need to know what parts need to come out and what can stay. I know that the mag block and upper need to go, but don’t know about the hammer, buffer & spring. Please advise.

If this is a legal SBR I wouldn't go changing the caliber, as it is printed on your tax stamp form.

Originally Posted by nubus

If this is a legal SBR I wouldn't go changing the caliber, as it is printed on your tax stamp form.

It is reg in many calibers, 5.56 beeing one of them.

I think you just need the upper, and maybe a different buffer. I am pretty sure most 9mm hammers will work with 223 uppers. If you already have another AR, try putting on the 556 upper from that gun and see what happens. I would dry fire and handle cycle it first to make sure your hammer will jive before you go live with it. I think the only problem you will run into is with the 9mm buffer being rather heavy it may not cycle with certain ammo, if not all ammo. All you would need to do would be drop in a regular or heavy buffer and you'd be set. Oh and get rid of the mag adapter, but thats kind of an obvious one.

This thread should make it clear as mud.
AR-15 from 9mm to 5.56???