Looking for a AR15 top assembly

[MUGEN]

I have a Colt HBAR with 20" barrel that comes with a carry handle.
I am now looking for a flat top. But could not see anything from Colt.
What is the alternative on the market? Are they all 100% compatible?

 

[Westicle]

you have to figure out if your lower is a small hole or big hole...

http://www.biggerhammer.net/ar15/uppers/

Small Hole vs. Big Hole

Colt owners often refer to having a small or large hole upper receiver (and corresponding lower receiver). Small hole receivers of .250" are the "milspec", but during early production by Colt for the civilian market, they produced rifles utilizing a .315" spec, so as to preclude use of military uppers on civilian lowers. This production lasted until roughly 1993-1994. As a general rule of thumb all Colt "unblocked" lower receivers, are large hole, while pre-ban "blocked" lower receivers represent transition years where occasional variations popped up.

(Above left) Armalite large hole upper receiver... This was produced in limited numbers, and is no longer in production. (Above right ) New Colt small hole upper as shipped with a HBAR upper assembly. (Below) Colt SP1 and A2 upper receivers.

 

[beltfed]

Buy a stripped flat top upper, and spin on your HB...

 

[MUGEN]

Thanks for your feedback.
I checked mine. The holes are both 0.250"