View Full Version : clearance between free float rail and shaved FSB
Androctonus
04-06-19, 20:43
Hello everyone. I got the FSB on my Colt 6920 shaved down and put a free float rail over the gas block. The gunsmith didn't shave the FSB down as far as he could. At the least, I can barely stick a 0.030" feeler gauge between the side of the gas block and the rail. There's more clearance on the other sides, as well as top and bottom. Is that going to be enough free float clearance when the M4 profile barrel whips under fire? The gunsmith refuses to shave the gas block any further.
It will hit the rail. I don't know what rail you have, options are another rail, shave down the fsb down yourself or a lower block. Common issue with some rails that are a wee bit narrow. Did he have the rail available when shaving?
Know for a fact that the clearance on the Centurion rifle is more than that and it will pop their brand of panels off, I had to move them. I've also had to shorten a Magpul screw on another brand.
Utilizing that now famous method from the French inventor Joseph Nicéphore Niépce and posting it here would greatly assist us in giving you an answer. It would also help to know which rail, as mark5pt56 mentioned.
Hello everyone. I got the FSB on my Colt 6920 shaved down and put a free float rail over the gas block. The gunsmith didn't shave the FSB down as far as he could. At the least, I can barely stick a 0.030" feeler gauge between the side of the gas block and the rail. There's more clearance on the other sides, as well as top and bottom. Is that going to be enough free float clearance when the M4 profile barrel whips under fire? The gunsmith refuses to shave the gas block any further.
I don't think knowing what rail he has matters. He either needs less gas block or more room. 30 thou isn't near enough room IMO.
Androctonus
04-07-19, 11:50
It’s a narrow BCM MCMR handguard. The gunsmith had it when shaving the gas block. I’d consider getting another rail if I didn’t like the MCMR so much. I’ve searched the forums and have seen 1/16th of an inch minimum clearance said several times. But, someone said 1/32 per side for 1/16th total, so I thought I’d ask. 0.030” is almost 1/32 (0.031”).
If you like the BCM handguard, then the best solution is to shave the FSB further. I wouldn't use that gunsmith again.
Androctonus
04-07-19, 13:06
Utilizing that now famous method from the French inventor Joseph Nicéphore Niépce and posting it here would greatly assist us in giving you an answer. It would also help to know which rail, as mark5pt56 mentioned.
Here's the best I could do, Iraqgunz. It's the top right of the gas block under the handguard's pic rail. The photo makes it look a little worse than it is.56755
justin_247
04-07-19, 13:28
Deleted.
Androctonus
04-07-19, 13:49
Deleted
By the looks of it, you need your money back on that hack job, is it even refinished? Take the rail off and if you have files or a Dremel to clean it up. I don't know what he charged, but this may be easier for you https://www.opticsplanet.com/midwest-industries-low-profile-gas-block-for-750-diameter-barrels-mctar-lpg-750.html
It covers the "front sight base scar" if that's important. I do know it will go under the MI rail of course. I used that on a BCM that has a FSB, red loctite on the screws(3) and ran at least 6-7k on it hard without issue.
A pinned, shaved down FSB is a pretty much the most robust gas block you can get, and I wouldn't replace it. Having cut down several FSBs myself (in exactly the way shown in the youtube video below), I suggest using a dremel or small grinding wheel (I think the dremel is a better option since you don't need to remove much and it'll be easier to control) to remove material until you have enough clearance. Then cold blue the parts you made shiny. Here are some links:
https://www.youtube.com/watch?v=5sBqDpi-Cb4
https://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/metal-bluing/liquid-cold-bluing-chemicals/3-oz-super-blue-sku167008200-24770-11060.aspx?cm_mmc=cse-_-Itwine-_-shopzilla-_-167-008-200&utm_medium=cse&utm_source=connexity&utm_campaign=itwine&utm_content=167-008-200
Finally, I don't think it's necessary to remove it from the barrel (taper pins can be a bear to get out). Just take the rail off and go to town on the high parts of the gas block. Getting it a warm won't hurt - gas block get pretty hot during use. Just be careful not to hit the gas tube or the barrel with the dremel...
Good luck!
A pinned, shaved down FSB is a pretty much the most robust gas block you can get, and I wouldn't replace it. Having cut down several FSBs myself (in exactly the way shown in the youtube video below), I suggest using a dremel or small grinding wheel (I think the dremel is a better option since you don't need to remove much and it'll be easier to control) to remove material until you have enough clearance. Then cold blue the parts you made shiny. Here are some links:
https://www.youtube.com/watch?v=5sBqDpi-Cb4
https://www.brownells.com/gunsmith-tools-supplies/metal-prep-coloring/metal-bluing/liquid-cold-bluing-chemicals/3-oz-super-blue-sku167008200-24770-11060.aspx?cm_mmc=cse-_-Itwine-_-shopzilla-_-167-008-200&utm_medium=cse&utm_source=connexity&utm_campaign=itwine&utm_content=167-008-200
Finally, I don't think it's necessary to remove it from the barrel (taper pins can be a bear to get out). Just take the rail off and go to town on the high parts of the gas block. Getting it a warm won't hurt - gas block get pretty hot during use. Just be careful not to hit the gas tube or the barrel with the dremel...
Good luck!
I love the guy but that video was painful to watch! Holding it with the rag is asking for a painful manicure. The kind that requires an ER visit to fix.
Not enough room based on what?
I don't think knowing what rail he has matters. He either needs less gas block or more room. 30 thou isn't near enough room IMO.
That picture doesn't do much for me. Removing the rail would have been better so we can see the Full Monty. On another point I get why people are obsessed with pinned gas blocks (especially when it was an FSB), but sometimes it's more prudent to replace the barrel or replace the FSB with a properly installed low profile gas block.
Here's the best I could do, Iraqgunz. It's the top right of the gas block under the handguard's pic rail. The photo makes it look a little worse than it is.56755
Androctonus
04-07-19, 18:12
Thanks guys. I took the upper to another gunsmith who shaved away the remainder of the A frame and cold blued it again. It's got about as much clearance as is possible now. I would've posted a picture with the handguard off, but I don't really have the tools and know how to do it. I thought about replacing the barrel with a Daniel Defense with a pinned low pro gas block, but I got a good deal on a whole DD upper instead. I've got a spare lower, so I'll have three ARs now. I should be pretty well equipped.
Is there a suggested amount of clearance between a rail (and rail mounted accessories) and the gas block?
I only have 2 free float rails, both are ALG. One is 13.5" and pretty rigid, the other is 15" and has enough deflection that I could get the screws for a MLOK bipod mount to contact the gas block. I just moved the mount enough that I couldn't force contact with reasonable pressure and drove on.
Androctonus
04-07-19, 23:43
Is there a suggested amount of clearance between a rail (and rail mounted accessories) and the gas block?
I only have 2 free float rails, both are ALG. One is 13.5" and pretty rigid, the other is 15" and has enough deflection that I could get the screws for a MLOK bipod mount to contact the gas block. I just moved the mount enough that I couldn't force contact with reasonable pressure and drove on.
Good call moving the bipod mount. I've read 1/16th of an inch clearance between the gas block and handguard is the accepted standard if there is one. But, I've read the minimum varies depending on barrel profile. I know my PWS upper has much less than 1/16th clearance all around the gas block, but it has a medium weight barrel profile.
Watch this video. You may want to rethink 1/16" being enough. https://www.youtube.com/watch?v=_9ULBtsnkR0
Needs to be 1/8 minimum all the way around the gas block.
A shaved front sight base is not compatible with a BCM rail. The BCM rail is too narrow.
Androctonus
04-08-19, 16:54
BCM didn’t say it was a no go. Also, ADCO shaves FSBs to fit under BCM rails. Would they do it if it wasn’t a good idea?
BCM didn’t say it was a no go. Also, ADCO shaves FSBs to fit under BCM rails. Would they do it if it wasn’t a good idea?
Did ADCO shave your FSB?
Androctonus
04-08-19, 19:26
No, but if I ever do it again, they will. Took 3 local gunsmiths to get it right. Hopefully it’s all good now. I had mine shaved down as far as the photo on ADCO’s website. You pick on their menu what handguard it’s going under and BCM KMR is one. MCMR is the same diameter.
Lots of bad ideas happen in the firearms industry. They do it because people ask for it.
BCM didn’t say it was a no go. Also, ADCO shaves FSBs to fit under BCM rails. Would they do it if it wasn’t a good idea?
Androctonus
04-09-19, 05:45
Lots of bad ideas happen in the firearms industry. They do it because people ask for it.
I’m pretty new to ARs, but this doesn’t surprise me. I’ll consider other options. If I re-barrelled, could I probably re-use my Colt bolt that has 1000-1500 rounds on it? Or time for new bolt and maybe cam pin?
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