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Posted: 10/11/2008 10:30:28 PM EST
In Glenn Zediker's new book The Competitive AR15, he covers many accuracy tricks for our beloved rifles. One that got me thinking was his mention of improving fit by shimming between barrel extension & upper receiver. Do any of you fellas out there do this for your builds?

Thanks -

Ben
Link Posted: 10/12/2008 7:56:39 AM EST
Why? Just tighten the barrel nut until you get gas tube hole alignment. Some times you may need to tighten & loosen a few times to get alignment without exceeding 80 lbs/ft torque. Worse case, just lap the front of the receiver a couple of thousandths on a glass plate with 400 grit wet-or-dry paper and oil.
Link Posted: 10/12/2008 8:12:17 AM EST
Actually that would have been a decent idea on the 629 I just put together. I had a bitch of a time getting the barrel nut to line up with the next hole so I could get the gas tube in.
Link Posted: 10/12/2008 10:32:28 AM EST
Shim the front of the receiver, or the diameter of the extension? The first times the barrel nut (as others explained above) and the second tightens the barrel inside the receiver.

Which type?
Link Posted: 10/13/2008 3:43:59 AM EST
i didn't even think about shimming the front of the receiver. I meant shimming the diameter--the cylindrical portion of the barrel extension...

Cylindrical..wow, try using that in a sentence today.

i'm guessing no one really does this. I sure haven't.
Link Posted: 10/13/2008 4:25:38 AM EST
These work


buckmeister
Link Posted: 10/13/2008 5:06:44 AM EST

Originally Posted By benz_004:
i didn't even think about shimming the front of the receiver. I meant shimming the diameter--the cylindrical portion of the barrel extension...

Cylindrical..wow, try using that in a sentence today.

i'm guessing no one really does this. I sure haven't.


That is what I thought. Yes, some do it for match barrels. What were you planning to shim with? A liquid like blue loctite is probably your best bet.
Link Posted: 10/13/2008 9:11:06 AM EST
How does shimming the barrel change the interaction with the Bolt/Carrier group? Does this affect headspacing in any way?
Link Posted: 10/13/2008 12:48:06 PM EST
You are only talking a couple thousandths of an inch, and the headspacing is determined by the barrel extension, not the depth the barrel & extension are seated into the receiver. The tolerances on receivers is probably more than the thickness of the shims involved.
Link Posted: 10/13/2008 2:27:22 PM EST

Originally Posted By Bretshooter:

Originally Posted By benz_004:
i didn't even think about shimming the front of the receiver. I meant shimming the diameter--the cylindrical portion of the barrel extension...

Cylindrical..wow, try using that in a sentence today.

i'm guessing no one really does this. I sure haven't.


That is what I thought. Yes, some do it for match barrels. What were you planning to shim with? A liquid like blue loctite is probably your best bet.


Agreed, the blue loctite works well for taking up any clearance between the barrel extension and upper. Makes for a more solid joint between the two and comes apart easily. That doesn't mean to apply it to the threads, but as long as one uses the blue, it isn't any big deal getting it apart with a little heat. However I prefer to use lube on the upper threads/barrel nut. Hey wasn't there another thread similar to this recently?
Link Posted: 10/13/2008 7:40:48 PM EST
Yup.
Link Posted: 10/15/2008 12:01:09 AM EST
Guess I missed it then fellas, I don't peruse here all that often. Thanks for all the input, your ideas & contributions to answering my questions is greatly appreciated.
Link Posted: 10/15/2008 12:14:10 AM EST

Originally Posted By LynC2:

Originally Posted By Bretshooter:

Originally Posted By benz_004:
i didn't even think about shimming the front of the receiver. I meant shimming the diameter--the cylindrical portion of the barrel extension...

Cylindrical..wow, try using that in a sentence today.

i'm guessing no one really does this. I sure haven't.


That is what I thought. Yes, some do it for match barrels. What were you planning to shim with? A liquid like blue loctite is probably your best bet.


Agreed, the blue loctite works well for taking up any clearance between the barrel extension and upper. Makes for a more solid joint between the two and comes apart easily. That doesn't mean to apply it to the threads, but as long as one uses the blue, it isn't any big deal getting it apart with a little heat. However I prefer to use lube on the upper threads/barrel nut. Hey wasn't there another thread similar to this recently?



I consider shimming & loctite-ing to serve two different purposes. Shims are added to make up for tolerance between (in this case) barrel extension & upper receiver. Measuring with a precision instrument such as a vernier caliper indicates the difference between inside diameter of the receiver & outside diameter of barrel extension. Dividing the difference in diameters by two provides you with something close to the thickness you need for inserting a shim. I don't know how much tolerance is needed for jamming a shim in between--this is half the reason I brought the topic up to begin with. I was/am seeking technical information not typically found by poking around on google. Next question is, what are the tolerances of upper & barrel extension? How good is 'good?'

Blue loctite is not a solid substance (like a shim is), nor does it dry to form a particularly solid substance. I don't trust loctite to stay where it needs to (around the mating surfaces) when one inserts barrel/extension into the upper. Someone convince me how this helps.
Link Posted: 10/15/2008 4:38:22 AM EST

Originally Posted By benz_004:

Originally Posted By LynC2:

Originally Posted By Bretshooter:

Originally Posted By benz_004:
i didn't even think about shimming the front of the receiver. I meant shimming the diameter--the cylindrical portion of the barrel extension...

Cylindrical..wow, try using that in a sentence today.

i'm guessing no one really does this. I sure haven't.


That is what I thought. Yes, some do it for match barrels. What were you planning to shim with? A liquid like blue loctite is probably your best bet.


Agreed, the blue loctite works well for taking up any clearance between the barrel extension and upper. Makes for a more solid joint between the two and comes apart easily. That doesn't mean to apply it to the threads, but as long as one uses the blue, it isn't any big deal getting it apart with a little heat. However I prefer to use lube on the upper threads/barrel nut. Hey wasn't there another thread similar to this recently?



I consider shimming & loctite-ing to serve two different purposes. Shims are added to make up for tolerance between (in this case) barrel extension & upper receiver. Measuring with a precision instrument such as a vernier caliper indicates the difference between inside diameter of the receiver & outside diameter of barrel extension. Dividing the difference in diameters by two provides you with something close to the thickness you need for inserting a shim. I don't know how much tolerance is needed for jamming a shim in between--this is half the reason I brought the topic up to begin with. I was/am seeking technical information not typically found by poking around on google. Next question is, what are the tolerances of upper & barrel extension? How good is 'good?'

Blue loctite is not a solid substance (like a shim is), nor does it dry to form a particularly solid substance. I don't trust loctite to stay where it needs to (around the mating surfaces) when one inserts barrel/extension into the upper. Someone convince me how this helps.


OK, I know what you are talking about now. That would be to enable one to insert a very thin washer type shim between the barrel extension and upper to enable one to "time" the teeth on the barrel nut to align the gas tube to avoid over torqueing the upper assy. I've never seen a need for that, I would just swith the barrel nut for another if it were to occur.
The blue loctite is just to make a somewhat more rigid joint between the inner and outer surfaces of the extension and upper. Some of the fits are a bit looser than others. Just a "in case" it might help rigidity step. No proof that it helps or not.
Link Posted: 10/15/2008 5:00:40 PM EST
How does one propose to prevent the Locktite, regardless of color, from MELTING at one of the hottest points on the rifle?

Locktite is wasted there.
Link Posted: 10/15/2008 7:52:54 PM EST

Originally Posted By A_Free_Man:
How does one propose to prevent the Locktite, regardless of color, from MELTING at one of the hottest points on the rifle?

Locktite is wasted there.


not sure that loctite would melt, but i don't think it serves any useful purpose there either. Shim stock, as in a very thin (a few thousandths) strip of metal, the width of the barrel extension, is what I am getting at...Shim stock is typically aluminum, brass, cold-rolled steel, or SS. I would get SS in this app.
Link Posted: 10/16/2008 6:15:21 AM EST
I would use brass, a bit softer and easier to bend around the curve. It probably doesn't matter.
Link Posted: 10/16/2008 8:04:19 AM EST
Link Posted: 10/17/2008 2:37:57 AM EST

Originally Posted By Bretshooter:
I would use brass, a bit softer and easier to bend around the curve. It probably doesn't matter.



a shim a few thousandths thick doesn't take much to bend no matter what the material. i agree though, probably doesn't matter.
Link Posted: 10/17/2008 2:51:12 AM EST
[Last Edit: 10/17/2008 2:52:02 AM EST by Gunner23]

Originally Posted By benz_004:

Originally Posted By Bretshooter:
I would use brass, a bit softer and easier to bend around the curve. It probably doesn't matter.



a shim a few thousandths thick doesn't take much to bend no matter what the material. i agree though, probably doesn't matter.


+1, from a workability standpoint, material type probably wouldn't matter at that thickness. I would opt for the stainless steel though, like you mentioned. Less tendancy to compress. Brass is soft and, although it's not likely, *could* theoretically lose it's shape over time. I would think that for this application, you'd want the hardest metal possible that is still workable.

Bear in mind that I know nothing about this, so correct me if I'm wrong about any of it. This is an interesting concept though, and I'd be curious to know if you see any actual gains if/when you do it.
Link Posted: 10/17/2008 6:28:02 AM EST
Good points. I was thinking ease of assembly with the part, and less scratching of the receiver interior. I would imagine deformation once in place and tighted would be non-existant. Deformation during the seating/tightening is what I would be looking for.
Link Posted: 10/17/2008 8:51:43 PM EST
i may have to try a little experiment once i get everything in hand. I'm putting together a varminter in 204 ruger. A Krieger barrel order has been placed with WOA, now we begin the waiting..Maybe I'll try no shim vs shim test. It will depend on the tolerance remaining in my setup. I have a RRA upper waiting for the barrel..
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