I have a Bushmaster 16" flattop carbine, and am considering replacing the factory handguards with either an ARMS #58 or #58Mod SIR system.

I know that both will fit a carbine with a standard front sight, but I have a 3-rail gas block with flip-up front sight, pictured below.

http://www.bushmaster.com/shopping/bmas/Images/yhm-9370k.jpg

My first question is will the standard ARMS #58 fit on my rifle?

Also, how about the #58 Mod?

Thanks!

The #58 is for a mid length gas system so it will not fit with your front sight.

The #58 mod may work, but I am not sure that your flip up front will work or not. It might, but when you flip it down, it may hit the top rail of the #58mod. Im just not sure.

A friend of mine uses a #45 SIR with one of Bushmasters flip up fronts, the one that fits over there milled gas block. His front sight hits the #45 when he tries to flip it down. All he had todo was turn it around so it flips forward instead of back and he is ok.

If you run into a problem, this may work for you.

Thanks for the info!

It sounds like the #45 SIR is the way to go then.  

The #58 is for mid-length rifles, you have a carbine, there is 2" too much length the a #58 for what you have.
The #58Mod was made for carbine length weapons like you have. They work with standard issue front triangle front sights, and the gov't issue PRI folding front sights.
To put a free float system on and still keep dovetails for attachment to a barrel looks contradictory to what free float is all about. Since I don't recomend attaching anything to dovetails that are also fastened to a barrel, espec. when you have free float, get the better PRI front sight, plus lesson the front wt. at the same time.
Good shootin, Jack

Im not really saying the #45 is the way to go. Im not saying that I know it will even work. I havent tried that gas block/front sight combo. In fact, if I had to take a guess, I would guess it probably wouldnt work. It may work if you turn the flip up front sight around so it flips to the front, but again, Im not sure. There may be an issue with the 3 rail gas block fitting? I just dont know.

However, if it did happen to work, it would work with any SIR system that is made for a standard carbine length gas system. Those SIRs would include the #45, #46, #50 and #58mod.