View Full Version : Gas block for 11.5" Colt FBI barrel
coolcraigster
06-11-21, 13:31
Almost done with this build just trying to figure out gas block situation. Using Colt 11.5 FBI barrel. Of coarse it has the FSB pinned and I know I'm probably gonna get beat down for this but I'm installing SLR adjustable gas block since I will be adding suppressor later. I've never had an issue with the SLR AGB. Question is I was gonna use a set screw gas block and dimple the barrel. Of coarse the barrel has the milled slots for the FSB pins and when I install my dimple jig the rear hole directly under gas port where I need to put dimple is half barrel and half pin slot if that makes sense. Should I just dimple for the front set screw and leave the rear alone or use clamp on? Gas block will be totally covered by handguard. Thanks
SLR used to make gas blocks with different setscrew hole spacing. Depending on which one you have going longer or shorter may get you off of the pin bore.
coolcraigster
06-11-21, 14:43
SLR used to make gas blocks with different setscrew hole spacing. Depending on which one you have going longer or shorter may get you off of the pin bore.
I'm pretty sure the rear set is still directly under gas port and the front set screw is moved. Not 100% but I will investigate. Thanks
coolcraigster
06-11-21, 14:52
Just checked with SLR and the rear set screw will still be in the same location and the front set screw is moved. So I will still be in the same boat either way.
You mentioned a clamp on. Do you already have the set screw gas block?
coolcraigster
06-11-21, 15:46
No I don't have a gas block for it yet. I wanted to get some advice before I ordered one.
Get the clamp on version. I'd want the tightest seal afforded by the rear screw, especially with a can.
coolcraigster
06-11-21, 20:27
Get the clamp on version. I'd want the tightest seal afforded by the rear screw, especially with a can.
Thats the direction I was leaning. It will be completely covered by the handguard so I shouldn't have to worry about it moving. I've never had a clamp on move before.
It should be fine under the rail. You may want to check inner dimensions to make sure it will fit inside your chosen rail AND with the accessories you'll be putting on it.
Combat_Diver
06-12-21, 01:07
Almost done with this build just trying to figure out gas block situation. Using Colt 11.5 FBI barrel. Of coarse it has the FSB pinned and I know I'm probably gonna get beat down for this but I'm installing SLR adjustable gas block since I will be adding suppressor later. I've never had an issue with the SLR AGB. Question is I was gonna use a set screw gas block and dimple the barrel. Of coarse the barrel has the milled slots for the FSB pins and when I install my dimple jig the rear hole directly under gas port where I need to put dimple is half barrel and half pin slot if that makes sense. Should I just dimple for the front set screw and leave the rear alone or use clamp on? Gas block will be totally covered by handguard. Thanks
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
coolcraigster
06-12-21, 01:36
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?
Sorry.. I read the original post wrong.
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.
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.
https://photos.smugmug.com/photos/i-4kNfDKZ/2/dba2c85d/X3/i-4kNfDKZ-X3.jpg
https://photos.smugmug.com/photos/i-wMcWSpf/2/dac42d91/X3/i-wMcWSpf-X3.jpg
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.
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.
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
06-12-21, 23:43
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
I would just use something like this to cover taper pins (and maybe BRT insert as MistWolf suggested)
https://www.midwayusa.com/product/1003405858?pid=159104:
65959
otherwise I would dimple at least one set screw.
coolcraigster
06-13-21, 07:51
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.
Yeah I thought about shaving down the FSB. I just wanted the ability to adjust when running with and without suppressor. Im still thinking about doing that. Just hate to do it to a Colt FSB...
Yeah I thought about shaving down the FSB. I just wanted the ability to adjust when running with and without suppressor. Im still thinking about doing that. Just hate to do it to a Colt FSB...
The HRT barrels are already pretty close to optimally gassed for mixed use, I’d personally just chop and use as-is.
Sent from my iPhone using Tapatalk
If shaving it down is an option you'll consider, it's a no-brainer. Re-purposing a FSB on a different barrel is tricky unless you have an outfit like ADCO do the install, so you might as well use what is fitted already. Bombproof, too. Then do the gp reduction kit.
Yeah I thought about shaving down the FSB. I just wanted the ability to adjust when running with and without suppressor. Im still thinking about doing that. Just hate to do it to a Colt FSB...
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