View Full Version : Gas Block Clearance
netchemica
06-07-13, 15:33
I installed a BCM gas block on my Rainier match barrel. I could not see day light between the block and my CMR rail so I filed it down a few thousandths of an inch at a time until I was able to see daylight between them. However, that gap is tiny, I feel I would have a hard time sliding a piece of paper through there.
Is there a spec I can go by? Will this tiny gap affect my accuracy at all?
I don't think having a space between your block and your rail would cause any accuracy problems?
netchemica
06-07-13, 20:05
I don't think having a space between your block and your rail would cause any accuracy problems?
I guess my question sounded a little weird. I was more concerned with how much space is required between the block and the rail. I have a very small gap between the two and I'm not sure if it's enough to not affect the harmonics.
Judging by some ar15 barrel whip videos I've seen, you'll need much more clearance.
I had the same issue with my 300blk build. My MI SS rail left very little clearance over my low pro bloc.
So far I haven't noticed any issurs but in all fairness I haven't shot it enough. Damn ammo shortage ;-)
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netchemica
06-07-13, 21:01
Judging by some ar15 barrel whip videos I've seen, you'll need much more clearance.
That's what I was thinking as well. Is there a "rule of thumb" / clearance specification that I can follow? I have it down to almost the set screws. Seems like I'll have to shave those down also. :(
You have less clearance than this?http://farm6.staticflickr.com/5460/8908323074_38859c97e6_o.jpg
I think Stick said he used a Vltor lopro gas block. The Vltor and BCM I have are virtually identical in size. Do you have a picture?
I've been trying to get an answer to this recently myself due to a troy/vtac rail that isn't sitting centered around the barrel. A highpower high-master i spoke with suggested that a folded over dollar bill was enough clearance. Im looking forward to hearing other inputs.
-rvb
netchemica
06-08-13, 10:02
You have less clearance than this?[IMG]http://farm6.staticflickr.com/5460/8908323074_38859c97e6_o.jpg[/IG]
I think Stick said he used a Vltor lopro gas block. The Vltor and BCM I have are virtually identical in size. Do you have a picture?
http://i.imgur.com/WWxpUHXl.jpg
That's after I ground it down a bit.
The barrel doesn't seem to align properly. When I installed the gas block set screw into the dimple, the gas block made contact with the rail near the gas tube. I disassembled it, used a ruler to make alignment marks and saw that the dimple wasn't aligning the gas block properly. When I align it with the marks, it sits right, but it still droops a little. Looks like the gas block has a lot of room on top, next to no room on the bottom.
Any end view shots? How tight is the gas block when you slide it on the barrel? The barrel is .750" at the gas block area, right?
I would thing a credit card thickness would be appropriate.. (pun intended) but what do i know.. Cheers.. Ron
netchemica
06-08-13, 11:36
Any end view shots? How tight is the gas block when you slide it on the barrel? The barrel is .750" at the gas block area, right?
Here is the top and bottom (click the image for bigger size):
http://i.imgur.com/eGkmDInl.jpg (http://imgur.com/eGkmDIn)
http://i.imgur.com/mF5XxqYl.jpg (http://imgur.com/mF5XxqY)
The gas block fits snugly over the barrel and comes off with a little force, but not to the point that tools are required. The barrel is .750 at the gas block (spot on according to my caliper). What's weird is that the gas block measures .742in between 3 and 9 oclock, and .752in between 6 and 12. Is that normal? (I doubt that has anything to do with the rail clearance, it's just intriguing)
That looks really close.. I use a brake cylinder hone if the gas block is too tight, so far all that I've had were too tight.. Ron
netchemica
06-08-13, 12:32
That looks really close.. I use a brake cylinder hone if the gas block is too tight, so far all that I've had were too tight.. Ron
The gas block fits fine onto the barrel, it's the barrel / rail alignment that is off.
Is there a procedure for checking alignment of the barrel? I'll borrow a machinist straight edge from work to inspect it the rail against the, but I don't know what I need to do to make sure the barrel is seated properly.
This is sitting on a Aero Precision upper receiver, are they known for being quality or could that be the problem?
edit: Scratch that, I had one in my garage that my friend left a few weeks ago. The rail is level when checked against the top of the receiver. Seems like barrel is not sitting flush. When I was torquing down the barrel nut, the vise block slipped out and the upper receiver took a 3 foot drop onto the floor ( :( ). It landed on the forward assist, but would the receiver bouncing be enough to offset the barrel? This is a bull barrel and I don't see any marks indicating impact anywhere on the barrel.
I understand.. I was referencing the "out of round" inside diameter of the gas block you mentioned. I saw that it went on OK; just kinds threw that in.
I thought the issue was the bottom of the gas block impacting the handguard/tube. A good pick of the muzzle that shows the handguard might show something.. Ron
I think you could just grind this down...
http://i.imgur.com/mF5XxqYl.jpg (http://imgur.com/mF5XxqY)
The drop shouldn't have done anything serious. I'd grind and move on. A 1/16 should be enough with a barrel that stiff.
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AlphaKoncepts
06-10-13, 12:23
I've been trying to get an answer to this recently myself due to a troy/vtac rail that isn't sitting centered around the barrel. A highpower high-master i spoke with suggested that a folded over dollar bill was enough clearance. Im looking forward to hearing other inputs.
-rvb
A dollar bill is the rule of thumb used for bolt actions when free floating around the stock.
AlphaKoncepts
06-10-13, 12:26
Here is the top and bottom (click the image for bigger size):
http://i.imgur.com/eGkmDInl.jpg (http://imgur.com/eGkmDIn)
http://i.imgur.com/mF5XxqYl.jpg (http://imgur.com/mF5XxqY)
The gas block fits snugly over the barrel and comes off with a little force, but not to the point that tools are required. The barrel is .750 at the gas block (spot on according to my caliper). What's weird is that the gas block measures .742in between 3 and 9 oclock, and .752in between 6 and 12. Is that normal? (I doubt that has anything to do with the rail clearance, it's just intriguing)
Top looks fine, bottom looks TIGHT. I'd consider milling down a few thousands from the bottom of the gas block on my milling machine.
Dammit, I was getting ready to order a CMR, but I keep seeing fitment issues. Does anyone have Vltor & BCM gas blocks sitting around that can measure them for overall height? I'd order a Centurion gas block, but those are no where to be found.
crosseyedshooter
06-10-13, 15:29
Have you confirmed that the CMR is aligned properly? Perhaps loosening the clamping screws and see if you could vertically realign the CMR. I imagine a couple thou shift at the mounting point would give you enough clearance at the gas block.
You could put a straight edge along the top rail and see if the CMR is tilted upwards.
Before grinding any more, I would rather confirm barrel and rail alignment.
netchemica
06-10-13, 16:24
I used a machinist ruler to confirm that the rail is aligned properly.
How can I check the barrel?
netchemica
06-10-13, 16:25
Dammit, I was getting ready to order a CMR, but I keep seeing fitment issues. Does anyone have Vltor & BCM gas blocks sitting around that can measure them for overall height? I'd order a Centurion gas block, but those are no where to be found.
I should have done that before grinding the block.
davidjinks
06-11-13, 07:39
Has anyone tried a LaRue low pro GB with one of these rails?
NewbieDave
06-12-13, 13:05
Here's my opinion... strictly an opinion based on why we free-float barrel. Keep in mind that I'm talking only about any type of handguard which only connects at the barrel nut area.
We free float AR barrels because we don't want to put extra pressure on the barrel as we grip the front of the rail system. If the guard is installed correctly, for example DD Lite & VTAC... the rail should be rock solid with little to no flex. That being said, if there is just a tiny bit of daylight between the block and inside of the rail... then the barrel is still free floating. Even if I was gripping the rail in such a way that would put extra pressure on the frontend... the rail should not be able to flex enough to touch the block. And if we are worry about barrel flexing, that doesn't happen until the round has left the barrel.. so it shouldn't affect accuracy.
My opinion, based on the info I gather above… is that as long as there’s day-light, I think you’re good. Best way to tell, take it out to the 100yd line and do some accuracy testing on sandbag. In all honest opinion… I’ve shot a standard 20” A2 out to 200 yards on sandbags using irons and the standard A2 handguard… it was just as accurate as my free-floated AR.
dc
davidjinks
06-12-13, 20:38
Deleted
Just another thought, but the Geissele SGB has a very low profile to it. I just installed the Nitride SGB and I was very impressed with the profile of it, I had the FSP shaved but it was kinda choppy and I was having the same clearance issue and it basically sat against the MK2 rail and I kept thinking is this eventually going to cause an issue, once I got the MK4 rails I had a bit more clearance but still wasn't comfortable with it, I decided that I wanted to go another route then the MK4 so while waiting for them I installed the SGB, after I find the rails I want to keep I do have the option to pin the SGB, the coil pin hole is already present which is a nice feature and comes standard in Geissele's SGB
dremel and grinding wheel is all you need. leave the top and take the bottom down some more. and round off those corners
A couple passes with an angle grinder was all I needed to gap the Rainier block. Touched up with a black sharpie, and all is good in my world again.
Kind of nerve racking for a couple seconds as the grinder spun up though...
Touched up with a black sharpie, and all is good in my world again.
Alumablack also works really well for touch up.
Is there a spec I can go by? Will this tiny gap affect my accuracy at all?
Is there a "rule of thumb" / clearance specification that I can follow?
A dollar bill is the rule of thumb used for bolt actions when free floating around the stock.
I had always heard that if you could slide a business card between the barrel and stock, then that was enough clearance... That was for a normal stock, where any tension from the sling would pull the stock down, away from the barrel. On an AR, where slinging up would pull the handguard down onto the gas block, I would want to maintain that much clearance under tension.
And yes, I think it matters... what's the point of installing a free float handguard if it's going to contact the barrel at the gas block?
crosseyedshooter
06-15-13, 14:59
Here's a good post (https://www.m4carbine.net/showpost.php?p=1672505&postcount=525) in the CMR thread regarding dimensions.
It's starting to look like most 0.625" low-profile gas blocks will fit without modification, but most 0.750" gas blocks may need some grinding.
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