I just received all the parts for my Grendel build so I am pretty excited. I measured the barrel extension diameter and compared it to the receiver and there is some play approx.002. I read that several people have used Loctite 242 or 243 to bed the extension. I went to my local auto parts store and could only find Loctite 242 and I picked up a feeler gauge set. The feeler gauge set I picked up had a gauge measuring .0015, so I unscrewed it from the set and I wrapped around the barrel extension which made it approx. 90% all the way around the extension. I carefully inserted it into the receiver and had to used a rubber mallet with light taps to get it in all the way. No more wiggle. I did lap the receiver to ensure it was square. My question is did I make a mistake by using the shim in place of the Loctite. I believe the shim is steel. I torqued the barrel nut to 60 foot pounds to line up the gas tub. I have a heavy 24 inch Shilen barrel, standard YHM receiver, free float tube, low profile gas block, m16 bolt carrier, rifle butt stock, rifle length gas tube and a 3lb trigger.
Bike Effects
Unregistered
I use Loc-Tite 242. I would worry about the shims moving. Loc-Tite is easy. My 5 shot group at 100 yards with my .223 upper was .260". Grendel upper under construction.
carbontetrachloride
Unregistered
I would not use a plain steel shim stock. It could create a corrosion nightmare should it get exposed to any moisture. I don't think there would be an issue with using stainless steel shim stock.I've used the blue threadlock sticks before and left them to cure for a few weeks before using them.
Would I have the same corrosion risk if the barrel was steel instead of stainless steel, If so then I might keep the shim.
All the steel parts are subject to the ill effects of corrosion, especially in high-humdity areas like the South, Eastern Seaboard, and Pacific NW. This is why I Cerakote my blasters, but you can't coat the extension and bore, so cleaning is more demanding in those climates.
If there is grease on the extension and shim, that should alleviate any corrosion issues. My main question with shims is what is the effect, if any, on lock-up with the bolt and extension, and can this cause off-axis load bearing of the lugs? Just an honest question, as I have never shimmed an AR15, but have bedded extensions with Blue Loc-tite.
Posts: 244
Threads: 20
Joined: Feb 2012
I would use green or red loc-tite and no shims even just the barrel against the upper could cause a corrosion problem.
[SIGPIC][/SIGPIC]
The bolt is matched to the barrel extension from Shilen and the lock up is smooth. With the shim I believe that it has a better chance of being centered in the receiver than without it. The shim wraps almost all the way around the barrel extension, only short by about 1/4 of an inch. I am not sure how much accuracy I will gain because of the shim but this was my best attempt. I did oil the shim, barrel extension, and receiver with Eezox prior to assembly. A bolt rifle action and barrel is screwed together so I figured it would be best to have a firm barrel extension and receiver connection and not just rely on the barrel nut. I am not an expert that is why I posted the question.
With lighter, shorter barrels, it's less of an issue and the torque of the barrel nut is usually enough to force the barrel extension flange against the face of the upper sufficiently.
With a 24" barrel, the preferred method is a tight-fit billet upper, or a bedded extension in a standard upper. Some guys will also sort through uppers until they find the tightest-fitting combo. I get more of a warm and fuzzy when I find extension to upper fits that are super-tight, but the trued face and trued barrel extension flange, combined with a true bore axis and trued extension will provide the foundation for an accurate gun. Lapping the bolt to the extension also helps, with a true bolt face.
So does Red Loc-titing the gas block once you have trouble-shot the gas system.
I had to order my upper from Midway so I did not have an opportunity to check the fit up prior to the purchase. In the future I will try buy local so I can check the fit up prior to buying it. At this point the shim I used has taken up the slack but it is still unclear if it will be better or worse than the Loctite. At least if I can easily remove the shim and replace it with Loctite if it doesn't work out. I am going to take it out next weekend and try it out and see what happens. I hope it is accurate and has no feeding or ejecting issues.
For what it's worth my opinion is see how it shoots if it is ok then leave it as it is. I would recomend the shim (mechanical) to take up the slack over Locktite (chemical). If you are not happy with the way it shoots then bite the bullet and replace the receiver with one that is a tight fit, this will also help with your confidence in your rifle.
I am going to take it out this weekend and see how it does. I will post the results . Thanks for the advice.
