Gas block for 11.5" Colt FBI barrel

[coolcraigster]

FYI when NSW Crane takes the FSB off for Mk12 gas blocks, they do the same thing you describe. (10.3" and 14.5" URG)

CD

So they still install the set screw gas block with the rear screw hanging halfway off where the FSB pin goes or are they modifying it somehow?

[GH41]

Sorry.. I read the original post wrong.

[Red*Lion]

I have been using clamp on adjustable GB on all of my builds for the past few years. I like Superlative Arms, Odin Works and J.P. Enterprise. All have served me well.

[MistWolf]

The SLR adjustable gas block is a good one, but the cross pin grooves complicates installation. Yes, you could use a clamp on. The problem with clamp on blocks is the angle of the cross screws changes as they're tightened, placing stress on the shank. This stress increases the chance of the screws breaking. I know there are guys who haven't had a screw yet break. But, it has happened to others.

I have an SLR adjustable gas block and it works well. Problem is, I no longer need it. My suppressed ARs are never shot without a suppressor. At first, I swapped suppressors around, changing the gas settings as needed, but it took just one range session to realize it's not worth the trouble. Since BRT introduced their Micro Ports and EZ Tune gas tubes, adjustable gas blocks are obsolete. The AGB was only useful when experimenting and once I figured out ammo, gas flow & buffers, I don't play with the adjustments any more.

coolcraigster, your barrel already has the best possible gas block- the FSB and it can be easily cut down.

I have a Colt 6933 11.5" upper. I removed the the FSB, installed a BRT micro port tuned for 100% suppressor use with 5.56 ammo and using an H2 or A5H2 buffer. I chopped the FSB and have been very happy with it ever since. If you don't want to use a Micro Port, use an EZ Tune gas tube.

[Clint]

This is the way. ^^^

[Red*Lion]

Yes, you could use a clamp on. The problem with clamp on blocks is the angle of the cross screws changes as they're tightened, placing stress on the shank. This stress increases the chance of the screws breaking. I know there are guys who haven't had a screw yet break. But, it has happened to others.

Those must have been some low quality cheapo GB, torqued the screws down too much or both. Never had a problem with a variety (4 different brands) of clamp on GB's.

[MistWolf]

Those must have been some low quality cheapo GB, torqued the screws down too much or both. Never had a problem with a variety (4 different brands) of clamp on GB's.

Perhaps. But the design stretches and bends the screw. At the same time.

As the screw is torqued, the clamp tightens around the barrel. As it does so, the angle of the screw holes change angles in relationship to each other. This bends the screw. The ends bend bows down and the middle bows up. The more you tighten a clamp on gas block, the more the screws stretch and bend.

When a screw is kept straight, the shank equally all around it's circumference. However, when the screw is bent, not only does the shank stretch, but the inside radius is compressed. Maybe the two equalize, maybe they don't.

But the outside radius stretches even further. Torquing the screw to the correct torque over stretches the outside radius and damages it. The screw will eventually fail.

Reducing the torque results in not stretching the screw correctly and it will work loose.

Now, subject the uneven stresses in the bent screws to the shock wave of each shot fired. When a screw of a clamp on GB breaks, it will be at the stress riser- the apex. Every time. Stress risers force stresses to concentrate in a smaller area, causing fractures.

Basically, the clamping screws of a clamp-on gas block are built in failure points.

[Red*Lion]

Perhaps. But the design stretches and bends the screw. At the same time.

As the screw is torqued, the clamp tightens around the barrel. As it does so, the angle of the screw holes change angles in relationship to each other. This bends the screw. The ends bend bows down and the middle bows up. The more you tighten a clamp on gas block, the more the screws stretch and bend.

When a screw is kept straight, the shank equally all around it's circumference. However, when the screw is bent, not only does the shank stretch, but the inside radius is compressed. Maybe the two equalize, maybe they don't.

But the outside radius stretches even further. Torquing the screw to the correct torque over stretches the outside radius and damages it. The screw will eventually fail.

Reducing the torque results in not stretching the screw correctly and it will work loose.

Now, subject the uneven stresses in the bent screws to the shock wave of each shot fired. When a screw of a clamp on GB breaks, it will be at the stress riser- the apex. Every time. Stress risers force stresses to concentrate in a smaller area, causing fractures.

Basically, the clamping screws of a clamp-on gas block are built in failure points.

I have never had any clamp on gas block do what you describe. If anything the holes start out being slightly angled up and out and come flush and straight when they are tightened down. Tightened, not cranked. No need to crank. At least that is my experience with them.

[Combat_Diver]

So they still install the set screw gas block with the rear screw hanging halfway off where the FSB pin goes or are they modifying it somehow?

Both set screw holes are dimpled. Rear dimple is over the taper pin hole.

CD

[alx01]

I would just use something like this to cover taper pins (and maybe BRT insert as MistWolf suggested)

https://www.midwayusa.com/product/10...58?pid=159104:

159104.jpg

otherwise I would dimple at least one set screw.