Sorry if this has been asked before. I couldn't fine any threads that answered my question with out confusing me more.

I put together an A2 upper with what I believe is a bushmaster barrel that has a carbine length standard FSB.

Am I going to run into problems zeroing?

Haven't had the chance to go to the range but wanted to know before hand if i was going to have any problems.

I also have a flat top upper with a carbine barrel and F marked FSB.  I have both a mil-spec and commercial carry handle. What would be the proper handle to use?

I prefer the finish of my commercial carry handle but from what I read I think it wouldn't work.  I also haven't taken this one to the range.

You should NOT have any problems sighting in with either the A2 upper using standard FSB on a carbine length gas barrel.  Nor should you have any problems sighting in your flat-top upper using commercial carry handle.  One of the members here who built many uppers with different configurations said there's just a lot of "internet chatter" about the use of F marked FSB vs. non F marked FSB on an A2 upper.  Just take both of them uppers out to the range and tell us how they run

I will try to go next week and report back.  Hopefully I wont run into problems using the RIBZ.

The difference between F and non F marked FSB's is the F marked is .040 taller. If you run out of adjustment get a .040 taller post. Last time I checked they were cheap.

I got one just incase. Going to shoot it first with the shorter one and if needed I'll add the longer post to the standard A2 FSB.

useing a F marked front sight with a a2 or a1 upper. you will have to crank the sight down. buying a taller sight post will make it worse. the sight base is .040 taller so if you had a .040 shorter post it would put the post the same as the standard post. you won't have worry about it anyway. the standard post will only be cranked down more.

For my A2 carbine build I have the non F marked FSB.   For my flat top carbine with commercial carry handle I have the F marked FSB

I bought the taller pin for the A2 build. However I don't think I will need it.