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View Full Version : Cutting FSB in favor of MBUS on Carbine quadsrails


Sweet Shooter
December 24, 2012, 08:35 PM
How crazy is this idea...? I'm tired of my left-leaning FSB how it has me drift my rear over to the left, and how it darkens the picture through my ProStaff scope. I'm a cuttin' it off and going to put a MBUS front sight on the carbine quadrail (which is actually square/plumb). The MBUS will still clear the scope objective lens. I only really shoot it with the scope. BUT. How much difference (shooting with the back ups) will it make if I do this... essentially shortening my sight radius by 1.5 inches?

Someone encourage me to cut this sucker off. It is my rifle after all... right?
-SS-

CTS
December 24, 2012, 08:39 PM
Wouldn't be the first one to cut that sucker off. Personally, I just replaced mine with a railed flat top gas block and then mounted the MBUS on it but like you said, it's your rifle.

Sweet Shooter
December 24, 2012, 09:09 PM
So you put the polymer on the gas block... did you warm it up yet? I'm just thinking I'll shorten the sight radius and mount to the rail.... still be okay for close stuff right?
-SS-

Ben Towe
December 24, 2012, 09:32 PM
My MBUS is on the gas block rail as well. If I ever find any ammo I'll get it hot and see how it fares. I'm thinking you'd have to get one pretty dang hot to damage it.

CTS
December 24, 2012, 09:44 PM
So you put the polymer on the gas block... did you warm it up yet?
My flip up sights are all steel from Yankee Hill Machine. I don't know about putting polymer sights on the gas block, maybe someone else here can advise you on that. I guess I confused MBUS with BUIS (back up iron sights).

Ben Towe
December 24, 2012, 10:00 PM
I confused MBUS with BUIS (back up iron sights).

They're the same thing. Magpul Back up Sights (MBUS). They are made of mostly polymer.

CTS
December 24, 2012, 10:50 PM
They're the same thing. Magpul Back up Sights (MBUS). They are made of mostly polymer.
Is that one of those cases of the same but different? One is polymer and the other is all steel. That is what I was saying is I mounted mine on the gas block but they are steel so I don't have any concerns about heat.

Sweet Shooter
December 24, 2012, 11:12 PM
So shall I cut it off or not?
-SS-

CTS
December 24, 2012, 11:20 PM
It's really your call. If you do and don't like the way it looks you can always replace it later with a low profile gas block or railed block. The last AR I bought doesn't have any front sight or anywhere to mount one. It is "Optic Ready" which means it has a rail on the receiver to mount an optic to and thats it.

Sweet Shooter
December 25, 2012, 01:53 AM
So Slappy, do you sometimes wish you had a front BUS?
-SS-

Metal god
December 25, 2012, 04:21 AM
I have the MBUS on my gas block . here is my little review on that .:)
http://thefiringline.com/forums/showthread.php?t=505325

CTS
December 25, 2012, 09:26 AM
So Slappy, do you sometimes wish you had a front BUS?
-SS-
Not really since I have several ARs. I have one set up for CQB with Red dot holographic scope and back up iron sights, my 6.8SPC has a scope mounted and I use it for hunting and this new one (optic ready) I picked up before the big panic started so I just snagged it. Still in the box.

5whiskey
December 25, 2012, 07:14 PM
I wouldn't cut the fsb off. Just replace it with a flat top gas block. It's your rifle. Having the fsb hacked off would irk me. Will your plan work? Well, yeah... probably. It'll shorten the sight base, but not by a tremendous amount. I just wouldn't like it were it my rifle.

Marquezj16
December 25, 2012, 07:40 PM
Have you seen what it looks like cut off? If the way it will look after does not bother you, then go for it. Me, I'd replace it with a railed gas block or a gas block with folding sights like the one below if you can find one.

http://www.midwayusa.com/product/814445/yankee-hill-machine-gas-block-with-flip-up-hooded-front-sight-and-bayonet-lug-bolt-on-mount-ar-15-lr-308-standard-barrel-750-inside-diameter-steel-matte

BillyJack3
December 25, 2012, 08:12 PM
I'd cut it off and get a longer, free float rail. 12" if that's an option. Then yes, mount the MBUS on the rail.

Ridge_Runner_5
December 25, 2012, 10:09 PM
I had the same problem with my AR. Drilled the holes for the taper pins a bit off.

What I did after a while was remove the pinned sight post and replace with a CMMG clamp on model. Nice and straight after that!

I would not mount your irons on the standard handguards. They aren't tight enough, and can wobble side to side, which will be much worse than dealing with a canted front sight base.

Heck, you can even get a clamp on gas block with a built in flip down front sight.

Mobuck
December 26, 2012, 07:43 AM
I've seen several FSB cut and finished to a nice, rounded profile. Looked fine in my opinion. As for the back up sights, I think it's BS unless you plan to go to war in a place where there is no chance of getting a resupply of optics.

Sweet Shooter
December 26, 2012, 09:09 PM
Okay the deed is done. I like it. I bought Midwest front sight instead of MBUS.
-SS-

CTS
December 26, 2012, 09:16 PM
A fine job you did at that!

Eghad
December 27, 2012, 07:32 PM
The front sight job looks good. What brand of drop in handguard is that?

Sweet Shooter
December 28, 2012, 01:45 AM
Eghad
The front sight job looks good. What brand of drop in handguard is that?

It's a 60 dollar UTG. I love it. It is rock solid and very straight—they clamp onto the barrel nut under the delta ring and around the end cap... very tight. A little heavy perhaps... if I were looking for longer quads, I'd get something better ($$), but for the carbine these work really well. I took it to the range today to sight in the new MI front sight and it took only 3 clicks from zero to sight it in... 9 rounds fired for three 2 inch groups at 100 yards. Much straighter than the 2A that I cut off.
-SS-

Dr Big Bird PhD
December 28, 2012, 02:52 AM
What type of front sight pin do you have? I have MI buis as well.

Sweet Shooter
December 28, 2012, 11:49 AM
Dr Big Bird... it's a 40 dollar KAC tactical cross hair with the adjustable bead. Personally I don't think it helps so much. Fun, but not necessarily an improvement. I might go back to the post.


-SS-