View Full Version : AR15 Upper changeout

October 28, 2012, 02:04 PM
I have a question about one of my AR rifles that I am changing the upper out on.

Colt Match Target HBAR with fixed carry handle. I have a new Colt flat top upper receiver for it. I want to change out the upper and switch between a scope with a riser and a removable carry handle.

My question is on the front sight, do I need a (Flat Top) sight or will the current FSB work with a detachable carry handle? I think it should be the same height but I have been unable to verify this through numerous searches.

I can go ahead and change the upper and leave the FSB as is, If it won't battle zero correctly with the detachable carry handle, then I will have my answer I guess.?

October 28, 2012, 04:05 PM
I have never tried it but I believe the front sight will work fine. Best way is to give it a try and see what happens.

October 28, 2012, 05:46 PM
The A2 front sight base is .04" shorter than a flat top FSB (F-marked). You may be able to unscrew the post enough to zero but it might wobble too much. If so Bushmaster sells a .04" taller post for about $6.

This is assuming you buy a milspec height carry handle, which I'd recommend. A Bushmaster or DPMS removable carry handle is A2 height so works with the A2 FSB but they're not flat top milspec height. It's much wiser to get the taller screw in post and be able to use 3rd party flip up rear sights and milspec carry handles.

October 28, 2012, 06:25 PM
I recommend leaving the A2 sight base and replacing the front sight post as Quentin2 suggests. tall front sight post $16 (http://www.midwayusa.com/product/810857/dpms-front-sight-post-square-extended-030-higher-ar-15-matte)

With a scope of 2x or more you'll never see the front sight, so don't worry about a folding sight. I've got several scope riser mounts and this one is my favorite. (http://www.laruetactical.com/larue-tactical-spr-m4-scope-mount-qd-lt-104)

October 28, 2012, 07:42 PM
You can definitely see the front sight at 2x mag. Even at 4x mag, there is still a shadow of the front sight post floating around in your line of sight.

October 29, 2012, 01:38 PM
Thanks for the info, I'll just go a head and change to a flat top FSB, I have the necessary tools and equipment to replace the FSB and ream the taper pin holes.

October 29, 2012, 07:56 PM
Well that would do the trick but really the taller post does just as good a job. A lot less work and cheaper too. I did this on an ArmaLite so I could use proper height sights. The standard post flopped around to get a 50 yard zero, the taller post did not.