The "F" marked FSB was for the flattop receiver.

I have a Colt Gov't 20" barrel with standard FSB.

Spec for elevation during a build is to leave the front sight 5mm high initially (see the TM).

With an A4 upper, where should I set a "high" front sight from Bushmaster, which I will install in stead of a new front sight base?

Or do I need  to change this with a 20" barrel?

Rear sight will be a larue.

It will probably be fine if you are using a 20" barrel.  Your front sight post may be a few clicks above flush, but I'd just go with it.  If worst comes to worst, you get one of those +0.040" front sight posts and go with that.  I'd bet you will be OK with what you have, though.

Quoted: It will probably be fine if you are using a 20" barrel.  Your front sight post may be a few clicks above flush, but I'd just go with it.  If worst comes to worst, you get one of those +0.040" front sight posts and go with that.  I'd bet you will be OK with what you have, though.

That's possible, but my Rock River (20" flattop) needed to come up quite a bit. Right now, the bottom ledge of the front site post is about even with the machined flat on the FSB. I measured it with my digital micrometer and it came out to .035" higher than the machined flat. In theory, my front site would be countersunk .005" if I got the Bushy front site and was shooting to the same POA.

It's kind of a crap shoot. You might need the taller front site, and you might not. If you go to the range and are shooting 6"-8" high at around 100yds, you'll probably need the taller front site post.

Hope this helps.

WIZZO

Thanks, guys. I was going to order from Bm anyway to get some moly-slide for barrel assembly, but if brownells has it I will order from them...