"F" marked sight tower

[ca_fireman19]

This may be a dumb questions, but what is an "F" marked sight tower? What does the "F" mean? Where would the "F" be on the sight itself? What is it if there is NO "F"?

[Jay Cunningham]

This may be a dumb questions, but what is an "F" marked sight tower? What does the "F" mean? Where would the "F" be on the sight itself? What is it if there is NO "F"?

Flat top.

It is properly installed on ARs that use the M1913 rail on the upper receiver. there is a little tiny F on the FSB if you look.

[ca_fireman19]

All I see is an "R1" inside of a circle on the right hand side.

[JonnyVain]

Did the barrel come with an A1 or A2 upper? Those don't use F marked.

[ca_fireman19]

As far as I know, the upper was a complete unit with a 16" profile barrel and "T" marked upper receiver. I was told it was off of a Colt lower that couldn't be imported into California.

[JonnyVain]

Just a little clarification, Cali, to help you on your journey.

A2 upper:


A1 upper:


A3 and A4 are flat tops.

F marked


The barrel length is 16" or 20" etc.

Barrel profile is the shape in terms of thickness

Government profile:


Heavy barrel


M4 profile

[ca_fireman19]

Okay, it is an M4 profile barrel. And it's on a flattop receiver...so will I have trouble mounting and zero'ing say an Magpul MBUS rear sight? What are the repercussions of using a NON "F" marked sight tower with a flattop and an MBUS?

[JBecker 72]

you have to buy a longer front sight aperture.
I think bushmaster sells one.

[ca_fireman19]

That kinda sucks...maybe I'm in a little over my head? Would it be better/cheaper/easier to buy an "F" marked sight tower or even railed gas block and throw a complete set of MBUS's on it?

[Singlestack Wonder]

The Bushmaster extended front sight post is simple to screw in and only costs a few dollars. Much easier than changing the FSB. It takes just a few minutes to remove the old post and install the new one.