I have an upper receiver and barrel whose fit is actually wobbly.  The barrel, when fully inserted into the upper receiver,  actually moves laterally. does that mean there is very limited potential for a precision build?

I have another upper into which the same barrel fits easily but there's no wobble.  I normally expect an upper/barrel extension to be a very tight squeeze, or requiring heat to get it in there.

the barrel's dimensions are reliable, a top quality mfg known for precision.

Thanks.

Joe Carlos seems to have a few things figured out:

Stabilize Barrel Extensions

"The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15" (March and April 2013) was the first topic I discussed in this magazine. I explained how the barrel fits in the upper receiver and how the barrel nut holds it captive. While the mounting system is simple and easy, it results in a lot of movement in the back of the barrel. One cure uses thin shim stock and the second a custom oversized BAT Machine match barrel extension. Both are locked in with green Loctite. Done properly, these will cut groups about a third."

LINK

Another good read HERE

I want at most -0.002" extension fit, but less than interference/thermal fit.

I too read some from Joe Carlos years ago and have since then always ensured a very snug barrel to receiver extension fit. I cannot always guarantee a receiver will be tight to the receiver extension, so I use shim stock and heat the receiver to require a press fit even with the shim stock. The result needs a soft-faced hammer to separate the two and with a fair amount of force. A WOA barrel upper of mine will maintain well under 0.5" groupings at 100 yards with 69 and 53 gr SMKs. I've not tried that barrel in a looser fit receiver to compare in a meaningful way, and with the results I'm getting I can't see a reason to even experiment without it. Previously I recall it not shooting as well as it does now, but I wouldn't call that hard data. Good BCM upper receiver groups generally also require a press fit at the least for most receiver extensions, so they're generally a good buy if you don't want to mess with shim stock.

Tight is right.

Attached File

I've been using .001" stainless shim stock on barrel extensions for years. Heat the receiver, fit the barrel, and it's locked in there (literally), but what's equally important is a completely square receiver face.

Recently, I tore apart one of my uppers. I had to put a reaction rod in the receiver and beat the end of it with a hammer for what seemed like 15-20 minutes to dislodge the barrel from the receiver.

Last year, I finally found a stripped BCM MK2 receiver, and it had a very nice fitment, but could have been tighter. I would rather see oversized barrel extensions versus undersized receiver bores, but God forbid anyone has to do any real work to assemble an AR. Nowadays, they stack together like legos.

This applies to complete uppers I purchase as well. When I purchased one of Geiselle's URG-I uppers back in 2018, I put it on the bench, tore it apart, made sure the receiver face was square, fit the Daniel Defense barrel, and reassembled the upper. Shoots like a laser...

I prefer an upper receiver that needs to be heated up in order to accept the barrel extension. I always true the receiver face prior to barrel installation.  

These two in unison makes for a solid straight rifle. Millions of AR-15's have been assembled without either.

I installed a 7.62x39mm barrel in a receiver that had an incredible level of slop. I stripped the interior of the upper with alcohol. I coated the barrel extension with Devcon Steel Epoxy and installed the barrel nut after cleaning all of the epoxy that flowed out.

I placed the assembly muzzle down and put a shop light close to the upper/barrel nut area to apply light heat to help it cure. The rifle shoots surprisingly well in spite of my low expectations.

Unless you are shooting match grade ammo most of this is wasted effort. FMJ ammo isn't capable of telling the difference.

Thank you everyone.

that's good shootin'