Front Sight Gas Block Question

calvin in keoma

calvin in keoma

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Somewhere near a grain elevator in southern Albert
I have a Colt A2 carry handle upper receiver and a Colt 20" pencil barrel on my AR build. And I have some questions regarding the front sight gas block.

What front sight gas block should I get?
Does it have to be a Colt front sight block?
Does it need to be F marked?

The barrel already has notches cut in it to attach a front sight.
 
Your first concern will be gas block size. If yours is a vintage barrel it will have a .625" barrel journal, and you will have to get a gas block that fits. In the mid 80s the A2 appeared, and with it the .750" gas block. Subsequently it became more and more common to have .750" barrel journals even on light profile barrels.

No reason for it to be a Colt. You do not need an F marked unit, in fact a non-marked would be more appropriate. The F front sight base was developed to solve sight in problems with the 14.5" carbines.

Your last problem will be with installation. At the factory, a front sight base and a barrel were mated together, then held in a jig and drilled. They were never intended to be separated and remated again. If you find a used front sight base that was drilled once, and try to mate it with your barrel that was drilled once, they likely won't fit properly. It will probably take a gunsmith who has a proper jig setup to clamp the two together, ream the holes oversize together, and install oversize taper pins.
 
A Caliper should do just fine. You’re looking for the nominal measurement. By the way, I believe the F marked FSBs are for Flat top ARs (those without built in carry handles)
 
WRH said:
A Caliper should do just fine. You’re looking for the nominal measurement. By the way, I believe the F marked FSBs are for Flat top ARs (those without built in carry handles)
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Flat top carbines specifically. Non-F marked FSP's for rifles regardless of upper style.
 
calvin in keoma said:
I measured it, it's .625"

So I should try to find a non F marked front sight gas block for a .625" barrel?
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That's correct! If it's not something you can do on your own, you'll also need to find a gunsmith who can set the barrel and sight in a fixture of sorts, and drill the front sight/barrel, and install the taper pins.

Has the barrel been previously drilled? You should see two half-moon shaped holes on the underside of the barrel, opposite the gas port.
 
Absolutely - especially given the barrel is already drilled. You'll need to measure the distance - preferably centre to centre of the two holes already drilled in the barrel, and then match it up and drill the holes in the new front sight - presuming it does not have holes already.

Caveat, the likely hood of matching an already drilled front sight to an already drilled barrel will be incredibly difficult. As stated above, the front sights and the barrels were drilled and pinned together, using a jig. I have only experienced it once, and with a lot of luck, where I could install a different front sight onto a barrel, with the holes lining up. The variances in drilling and pinning a front sight doesn't typically lend itself well to matching up an already drilled sight.
 
Brownell’s has new .625” FSB for $20. Out of stock at the moment though.

Buy one and send the barrel in to TacOrd.
 
Leg said:
Brownell’s has new .625” FSB for $20. Out of stock at the moment though.

Buy one and send the barrel in to TacOrd.
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Do this. Casey at Tac Ord is one of the few gunsmiths I’ve heard of being willing - never mind able - to do this job. You’ll have to wait, but it’ll be done right.
 
I highly recommend Tactical Ordinance. They just recently did my barrel and front sight base (in my case a 20" Faxon Gunner which wasn't drilled and a new FSB) in a very short turnaround time for a reasonable price (I believe it was 125$ if I remember correctly) and did a great job on it. Just remember to send in the barrel nut and front handguard retainer so they can be put on the barrel before the FSB is installed.
 
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