AR15 parts dimensions question

[caspian guy]

Does anyone know what the outside diameter of the barrel extension and the inside diameter of the upper should measure (and their tolerances)?

I am in the process of building another AR-15 upper. When I test fit the barrel to the upper there was more slop than I was used to. I measured the OD of the barrel extension at .998". I then measured the ID of the upper to be 1.002". So there appears to be about .004" of slop between them. I know that doesn't sound like much but it lets the barrel wiggle more than I am used to.

My normal fix for this would be to coat the outside of the barrel extension with blue loctite and assemble (making sure to wipe any extra loctite off so I can get it apart again if I have to). The idea being that when the loctite cures it will take up some of the slack so that the zero doesn't wander when it heats up.

Thoughts as to which part is off or is this just a case of tolerance stacking? Will the loctite bridge that much of a gap or will it wander on me as it heats?

Edited September 29, 2010 by caspian guy

[Ron B]

Try this link for the upper blueprints:

http://www.scribd.com/doc/14939212/Ar15-A3-style-Upper-receiver-blueprints

Shilen Barrels has extension dimensions here:

http://www.shilen.com/ar15Barrels.html

[Ron B]

To eliminate the gap between the barrel extension and the inside of the upper you can cut a piece of SS rolled shim stock to fit over the circumference of the barrel extension (leaving a gap for the locating pin). Probably .001 stock would work, .002 might be too tight. You will need to tap in the barrel. If you ever remove the barrel you will need to pound it out from the extension end.

This is the type of shim stock:

http://www.msdiscount.com/columnar.aspx?cat_id=2096&session_id=842972174&category_site=STARTOOL

http://www.lyonindustries.com/index.php?page=shop.browse&category_id=3&option=com_virtuemart&Itemid=3

[GentlemanJim]

I like to see the bore at 1.000, thats +or- zero

High temp RTV as a film on the barrel extension will help prevent the barrel from walking

I have seen as much as 1.006 in some over the counter recievers...and have been told of worse

The etch and anodize process can be the culprit in many cases.

LOTS of out of spec and poor quality parts are in the market right now, and will be for quite some time

The AR15 frenzy of a few months back spured the manufacture and import of some pretty ugly stuff

Much of which in indistinguishable visually from quality parts

Its best to stay with known quality manufacturers

Jim