Hi all,

I'm looking to purchase of a new upper for my AR, and even though long handguards are the rage these days (I have one too) I'm considering going a little old school with a fixed FSB. Specifically I'm looking at a 16" mid-length LW Centurion Arms complete upper with the fixed FSB and their new C4 M-Lok cutout rail. I plan to use an Aimpoint T2 and Vortex Micro 3X magnifier on this upper.

I was wondering what are the pro's and con's of the fixed FSB these days? Are they pretty-much obsolete give the fact that I could mount a DD fixed A2 front sight on the rail anyway? Will I miss the space provided by a 13" handguard and feel cramped? Limited light mounting options? Will the fixed FSB just annoy me when shooting the red dot even if I use a lower 1/3 mount and the rear BUIS is folded down? Any benefits to the fixed FSB besides ruggedness and cool looks?

The other option is getting this upper with their new 13" CMR M-Lok handguard, which honestly will likely be the direction I go, but I was curious what people's opinions are these days of fixed FSBs. If you do use them, what is your use case that you find them valuable? Thanks for the insights.

Chris

I have them on a handful of AR's still. Various 16 and 20in. Most of them also have removable carry handles.

The rifles I built for my kids and serve a 2 fold purpose. one, they'll learn to use iron sights. Two, any red dot I put on them today will surely be severely outdated by the time they take physical ownership.

My 16" mid-length has one, but mostly because that was what was available on a budget when I built it.
Also didn't have the budget for a RDS at the time, so irons was it for a while.

I still have one rifle with a fixed FSB, only because it ain't broke, so I don't need to fix it. But none of my others do, and I wouldn't buy a new AR with one (unless you specifically want a retro clone or something).



They still work just fine if that's what you have. And you can sometimes save a little bit of money on an upper by going with it. But it does limit your accessory options.

quote:

Originally posted by Chris17404:
Will the fixed FSB just annoy me when shooting the red dot even if I use a lower 1/3 mount and the rear BUIS is folded down?

You barely even notice it when shooting with a red dot.

I have an A1 clone and an A2 Colt that both have FSBs and fixed carry handles on them. My M4 clone originally had a FSB but I had that replaced with a low-profile gas block and a new handguard because with my aging eyes a standard 20" FSB is about the shortest that I can keep in focus. It has flip-up BUIS on it along with both a RDS and a scope, the latter two being swappable on QD mounts. My latest build is strictly a scoped rifle.

I shot the M4 with the FSB and a lower 1/3 cowitnessed RDS for a couple years. You quickly get used to the FSB being there and your brain starts to just ignore it. It doesn't interfere with the RDS at all. If you should decide to put a scope on it, anything over about 3x will make the FSB completely disappear.

I have 3.

Two are Colt Uppers (6933 & 6991) and the other is an Aero Precision Midlength upper I bought to make a training rifle / steel ammo whore. All use RDS.

I have 2 other Colt uppers that came with FSB shaved down by the factory as well with rails that cover the gas block.

Biggest (+) I can see is a more sturdy gas block and front sight.

Biggest (-) would be obscuration of a magnified optic and difficulty changing rails / placing accessories, etc. With a RDS setup you forget its there (if you shoot like you should, with both eyes open).

I just built a fixed front sight upper. 16" mid length gov't profile barrel, flat top upper, currently has B5 slim m-lok handguards and a Magpul rear BUIS. Haven't decided on an optic - might try a Trijicon MRO. Front sight is a non-issue with a lower 1/3 mount.

Idea is to be simple, light, and tough.

Long slim full length m-lok rails are great for ergonomics and accessories - I built a couple like that too - 16" mid and 10.3" carbine.

That said, I hate 16" barrels with carbine gas.

To me its all about the gas block attachment. If I get a full taper pinned FSB versus some silly set screwed nonsense I'll take the FSB every single time and deal with its issues. Reliability is my top priority. To your questions with an optic it doesn't bother me at all. If I have a choice I would get a lopro pinned gas block as the situation on hand-guards improves dramatically. But that "ruggedness" benefit is pretty much what drives me.

Yup...

Thanks for the replies, guys. Good insights. And I'm happy to hear that using a RDS with fixed front sight is no big deal once you get used to it. I have some more thinking to do, and I'll keep you informed on progress...

I no longer have a rifle or upper with a FSB. But not because the FSB has annoyed me with its presence within an optic's sight picture; I got used to that early on and to me it's a non-issue. Truth is, even with folding sights I tend to like having the open sights and red dot always cowitnessing. Call it a psychological crutch that I don't mind having in front of my eyes.

These days I have mostly piston ARs and for those gas impingement setups I still have (all in 300BLK) I've come to appreciate adjustable gas blocks now that I use a suppressor for as much as I do. But I am in the midst of building an A2-style rifle, so shortly one with a FSB will be back in the fold.

Still have 3 with the FSB on them, never notice during shooting.

Both my ARs have FSB. I'd prefer a folding front, but not enough that I've gone through the trouble of changing them. If I bought a new rifle or upper it would not have a FSB.

I have a few. Have a few lo pro also. All about what you want to do with it. Do you want rail space out further, etc?

The one thing I like is they are generally tough as nails. That's not lost on me. Otherwise I tend to look for pinned low pro GBs as well.

I personally really like the simplicity of the M4 carbine even in this modern day. Carry handle and all. Sure I have red dots and everything else, but I don't need batteries for a carry handle and I've never had one come loose.

Still a favorite of mine.

Yep at my agency all rifles are fixed sighted with A2 rear sights. Works for us...VI

quote:

Originally posted by hrcjon:
To me its all about the gas block attachment. If I get a full taper pinned FSB versus some silly set screwed nonsense I'll take the FSB every single time and deal with its issues. Reliability is my top priority.

Ding ding ding, what does he win Johnny?

Absolutely this. A proper pinned on gas block always beats out the nonsense set screw bullshit. I cant even tell you how many times Ive seen gas blocks come lose, rotate, etc and the guns become single shots.

True. A properly pinned fixed FSB is preferable to a shoddily installed low profile gas block.

But a properly pinned low profile gas block is preferable to a fixed FSB.

(Barring something like building a specific retro clone, as previously mentioned.)

quote:

Originally posted by RogueJSK:
But a properly pinned low profile gas block is preferable to a fixed FSB.

Yep

Honestly, carbine FSB on 16" barrels are what kept me away from Colt AR-15s for a long long time. Only when they did some shaved FSB with Centurion rails over them (6920-R Trooper, 6960 CCU) did I get remotely interested in them.