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Any AR builders/tuners — Lapping Q?

While I do believe that that the receiver face should be square as fundamental part of the design, I also think the receiver inlet for the barrel should also be the same size on all receivers. I find that from receiver to receiver, this dimension varies quite a bit (probably to a larger degree than the face out of square). if the tool has a pilot with a loose fit its going to be hard to lap something square by hand and can potentially give a false sense of of accomplishment.

It's hard to measure before starting and most estimate a measure based on lapping results - the look of the material that it took off of one side of the receiver rather than another measurement. I'm not saying it can't have positive outcomes or value, but just not for me.... I did try it twice but no real impact.

I did find that loctite on the extension gave better results for me when the fit was fairly sloppy.

I think the best bet for AR-15 accuracy is starting with a quality barrel .... so many AR's have been built with inexpensive mass produced barrels that are not built with accuracy in mind. I also think that barrel torque ranges can have an affect on the accuracy.
 
FWIW: White Oak Precision barrels (precision, not armament) have an extension that's 1 thou oversize.

And BCM uppers are 1 thou undersize.

And like someone else said, having positive lock up between the barrel and upper is the more important part.

Loctite 620 is the stuff you want to use, if you ever want to remove the barrel.
 
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I would like to be able to remove the barrel without getting the torch out and do understand the difference in locktite IE red , blue , green and purple... I use the purple for alot of things and like it because you can get it apart again easily.... when you say low strength locktite what do you mean....
So, that's a great question - because different experts have recommended different ones.

Basically, Loctite comes in different strengths - low, medium and high.

I have used 222MS with good results over 5-7 uppers.

Going forward, I was planning to use 220 because an expert recommended them. [Can't remember why it was suppposed to be better than 222MS]

Someone did recommend 243 because it's supposed to be 'wicking'. But, I just now saw it's medium strength,

For disassembly, with 222MS I have inserted a delrin 22 rimfire bore guide into the upper and whacked it good a few times to break the barrel loose.

More recently, I bought a heat gun and temp probe to heat the upper around where the extension is to ~ 200* F. It breaks loose with less effort. I'm going to try 243 on the next upper - thinking/hoping that the heat gun will loosen it enough.
 
FWIW: White Oak Precision barrels (precision, not armament) have an extension that's 1 thou oversize.

And BCM uppers are 1 thou undersize.

And like someone else said, having positive lock up between the barrel and upper is the more important part.

Loctite 620 is the stuff you want to use, if you ever want to remove the barrel.
Why 620 over, say, 220?

Thanks
 
IF your lapping tool fits the bore of the receiver (thermal fit uppers might be too tight to fit and you will need an undersize lapping tool)
a 0.001" undersize lapping tool, about 6" long will have a lapping plane "wobble" across the 0.5" radius of about 1/12 of a thousandths.
Example: Bore is 0.9995", tool is 0.9985", wobble at receiver face will be about 0.000083" :)
A true lapping surface helps but even a single point cutting/lapping works with a good fit with the tool.

I measure receiver bore for the first inch, measure extension, Spray on shim and let it cure, measure and burnish down to about 0.001 - 0.002" interference at room temp (using 7075 and 416 expansion coefficients).
Heat receiver in oven, barrel in freezer.
By the time I get the two parts going together the temperature difference is a little over 200F.
Slip together and hand tighten a nut.

If you doubt your lapping tool, check it.
Come on, we check runout on loaded cartridges.
V block, shim and indicator.
BCA-upper.jpg

New-22N-Barrel.jpg
Other than using a hammer it might help to measure interference (receiver bore ID/extension OD) of a "thermal fit". Tapping it in for the last 1/4" just means the rest of the fit slips and might be loose. While Blue Goo fills up voids in the fit, a true thermal fit just might be more stable.

Upper-Bore-1.jpg


the most important area to fit is the heavy wall of the receiver, not just the nose.

AR-Barrel-Support.jpg


22NOSLERSHIM.jpg
 

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Why 620 over, say, 220?

Thanks
There are 5 different Loctite retaining compounds. 620 is most commonly used and has the highest heat rating at 450*
Most are green in color. 638 is another one to consider, it differs from 620 on cure time, heat rating, and it is oil resistant.

blue and red loctite are not the correct tools for the job.
 
There are 5 different Loctite retaining compounds. 620 is most commonly used and has the highest heat rating at 450*
Most are green in color. 638 is another one to consider, it differs from 620 on cure time, heat rating, and it is oil resistant.

blue and red loctite are not the correct tools for the job.
Thanks. What's wrong with blue? E.g., 220, 222, 243?
 
⁷⁹8⁷
Thanks. What's wrong with blue? E.g., 220, 222, 243?
Like Aaron said, it may not be what the mfg recommends but like many things, it'll work too. I've used it but I use 609 mostly, for this job. Still not what's recommended but it works great. Sometimes we over think things and sometimes, well... the opposite is true. In this use, it's really just taking up space more than anything, so a sleeve retaining compound seems like the logical choice, since I've never seen AR15 bbl retaining fluid marketed...yet. Lol!

FWIW, some people use it to hold rimfire bbls into the action. Nothing else. No threads, no clamp.....just 609 or similar. Apparently it works too, but I refuse to do it.
 
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