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Posted: 9/28/2013 8:25:09 PM EDT
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http://s876.photobucket.com/user/tbuckus/media/IMG_0030_zpsb37ea2ea.jpg.html?filters[user]=137654377&filters[recent]=1&sort=1&o=0#/user/tbuckus/media/IMG_0030_zpsb37ea2ea.jpg.html?filters%5Buser%5D=137654377&filters%5Brecent%5D=1&sort=1&o=0&_suid=138042863711806504778390458579I have a new AR that has a free floating forearm. I do not know the specific name since it came as a complete upper.
I noticed after about 200 rnds, the forearm started moving as in twisting about 1/8-1/4". The locking screw is tight, and the delta ring seams to be tight also. I am not sure what I am missing here. Below is a pic of the style that I have. Any advise would be appreciated. http://i876.photobucket.com/albums/ab328/tbuckus/IMG_0029_zps8aeed59c.jpg http://i876.photobucket.com/albums/ab328/tbuckus/IMG_0030_zpsb37ea2ea.jpg |
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yep, older YHM "ultralight?" rail.
its either the barrel nut being loose, or the jam nut came untorqued. the gas tube on the YHM feeds through the barrel nut, so there is a very small amount of room to move. unfortunately its hard to tell you which of the two it is over the internet. so remove the gas tube. properly torque down the barrel nut (3x tighten and loosen of 30ft lbs, then crank it to the first gas tube opening), thread the jam ring on with a little bit of Loctite, thread the rail on, and hammer the jam nut in place. |
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Most free-float tubes lock onto the barrel nut. The barrel nut can't get too loose, because the gas tube sort of half-way locks it in place, but it can be loose enough for the front of a tube like yours to feel like it's moving a lot.
Where did you get the upper? Can you go back to them and have them look at it? Otherwise, I think you need an AR-savvy gunsmith to check it out. |
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Cheap rail without an anti-rotation feature.
I suggest a replacement. Centurion C4 rails are good, and have flush cups on them too. |
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Quoted:
Cheap rail without an anti-rotation feature. I suggest a replacement. Centurion C4 rails are good, and have flush cups on them too. the anti rotation features is the set screw on the rail which stops the rail from rotating on the barrel nut (and if it does, it damages your barrel nut threads). the gas tube stops the barrel nut from rotating (much). neither solution is great but they are "anti rotation" features none the less. |
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Quoted:
the anti rotation features is the set screw on the rail which stops the rail from rotating on the barrel nut (and if it does, it damages your barrel nut threads). the gas tube stops the barrel nut from rotating (much). neither solution is great but they are "anti rotation" features none the less. Quoted:
Quoted:
Cheap rail without an anti-rotation feature. I suggest a replacement. Centurion C4 rails are good, and have flush cups on them too. the anti rotation features is the set screw on the rail which stops the rail from rotating on the barrel nut (and if it does, it damages your barrel nut threads). the gas tube stops the barrel nut from rotating (much). neither solution is great but they are "anti rotation" features none the less. Kind of... but not like they should be. I like how the HK416 anti rotation feature works! |
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Quoted:
the anti rotation features is the set screw on the rail which stops the rail from rotating on the barrel nut (and if it does, it damages your barrel nut threads). the gas tube stops the barrel nut from rotating (much). neither solution is great but they are "anti rotation" features none the less. Quoted:
Quoted:
Cheap rail without an anti-rotation feature. I suggest a replacement. Centurion C4 rails are good, and have flush cups on them too. the anti rotation features is the set screw on the rail which stops the rail from rotating on the barrel nut (and if it does, it damages your barrel nut threads). the gas tube stops the barrel nut from rotating (much). neither solution is great but they are "anti rotation" features none the less. That part in bold, that somehow the set screw will damage the barrel nut threads...... It doesn't work that way, the set screw fits into a recess in the barrel nut and basically locks the barrel nut and handguard together. Still not a true "anti-rotation" feature to my way of thinking, but nonetheless.....I think it's self-explanatory if you look at the barrel nut: http://yhm.net/index.php?main_page=product_info&cPath=2_4_51&products_id=32 |
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kbuc: Can't tell from the pic who made the handguard, but it appears to use the MI/YHM method of threading the HG tube over a proprietary barrel nut (no delta ring involved), then securing with a jam nut. If its just a loose jam nut that is causing your problem, its an easy fix. It helps if you have the right tool, a wrench with a hook on the end to catch the little notches in the jam nut, see link below. You could probably make one with a grinder and a 2" wide piece of strap iron.
Unscrew the HG tube enough to expose some of the threads on the barrel nut. Degrease the threads on about 1/2 inch of the rearmost threads on the barrel nut. Apply a few drops of blue thread locker to the degreased threads. Screw the HG tube back down to where the top rail touches (or almost touches) and aligns with the flattop receiver. Hand tighten the jam nut. While holding the HG tube in place with one hand, tighten the jam nut the rest of the way with the hooked wrench. About 20 ft/lbs of torque are sufficient since the jam nut is steel but the barrel nut is aluminum. Once everything is lined up and tightened, and the threadlocker has cured overnight, your handguard will not move until your loosen the jam nut. No need for the extra locking screws, they are more trouble than they are worth, IMO. good luck - CW http://yhm.net/index.php?main_page=product_info&cPath=2_4_51&products_id=30 |
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That handguard appears to be a MI.
They've typically used the single locking screw at the bottom of the rail where YHM has either had 2 (0300 and 0900) or none at all (earlier generations). YHM also never put holes in the 45* area for the "beauty ring" that this one has. Take the upper to someone who can disassemble it and check everything. Perhaps a call to MI is worthwhile to determine if they can verify it is their rail system and if so they'd be able to help you out. Where did you buy it? |
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The hand guard is not ours, it does not show our logo on the sight side above the barrel nut, the markings are different, and we never chamfered the trim ring holes.
What you are describing if consistent with a loose barrel nut, if the hand guard moves about 1/8" side to side the nut is loose and the gas tube is preventing the hand guard from unscrewing. I would suggest taking the upper to a AR friendly gunsmith and having them remove the hand guard, then correctly install it. Thank you Andy MI |
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