Gunsmithing AR barrel/upper fit with shim stock (how little shim is too little?)

[ToddM]

So I've read a lot about using shim stock to improve upper receiver to barrel fit, however when I've tried it I always run into issues. In theory by the raw ID/OD measurements it's possible I might snug 0.001 shim stock in there, but it never works, it kinks, bends, gets crooked as the barrel is going in, or all 3 etc. Now I could get a shorter section of 0.002 or 0.001 in there, especially if I warm the upper. However, does say putting 1-1.5" of shim stock centered on the top/bottom of bore do more damage than good? When it's that close would bearing retaining compound just be a better option?

Whenever I've seen it mentioned to use the shim trick it's always talked about putting it all the way or 3/4 of the way around, so if you can't get the shim stock that far around is it better to just skip it?

 

[GH41]

My opinion... If you are within 2K just glue it in. Even then it would be the very last thing I would do. Right after a $500 barrel and $300 trigger.

 

[LtDan47]

I hold the .001 shim in place with a cable. It keeps it flat and tight. I cut the cable tie just before the receiver threads touch the tie. Works pretty good.

 

[ToddM]

That's actually a pretty slick idea with the cable tie, I've also heard of guys running the shim a bit over the edge of the extension so it can be guided in better than trying to expect that it won't move at all.

 

[striped1]

Haven’t tried the cable tie trick but will fo sho. I freeze the barrel and heat the upper with a blow dryer or heat gun.

 

[whatsupdoc]

I have done this, easy. I cut a steel shim with a scissor until it fit exactly around the barrel extension with a tiny gap where the two shim ends meet.

I cleaned and de-greased the barrel extension and shim. I used Locktite 603 sparingly around the barrel extension wrapped the shim around the extension and held it in place using a 2 piece aluminum 30mm to 1 inch scope ring adapter. It fits and holds perfectly.

After I got it all together I used rubber bands to hold it until the Locktite cured. Warm weather it cures in an hour or so.

The shim bonds to extension and you are good to go.

Couple of sugestions, before you apply the Locktite test putting it all together. Make sure you wipe all excess Locktite so you dont glue everything together.

 

[ToddM]

Yeah, I've done it where there's enough of a gap to get shim stock most of the way around the extension, my question is more if you can't get it all the way around because it's already fit really close, at what point do you either just use say bearing retaining compound, or do you cut just a short say 1" shim and use that with bearing compound. Or in better words perhaps, is a short section of shim that does not go around most of the extension hurting more than help.

It's like the heating/freezing, you can make a lot of "room" doing it but you only have a couple seconds before that cold extension "closes" most of what you gained, and you can quickly cook retaining compound to sludge. So there again, lets say you freeze/heat and that gets you enough room to get say 1" of 0.001 shim stock in, does it hurt you because it's not supporting the whole extension evenly or does anything more solid in there even if it's just a short section help.

Certainly the better option is to have an extension milled that you can just barely get together with heat/cold and not use shim stock, but that ship has sailed.