AR barrel extension to upper fit issues

[doverpack12]

I have an AR barrel that didn’t shoot well but is a relatively expensive barrel and generally considered accurate. When installing, the barrel extension got extremely tight in the upper for the last 1/4-1/8” to the point I had to put the barrel nut on and use that to seat the barrel then backed off checked and torqued.

Well after 50 plus rounds and 3 different ammo varieties it’s not shooting as well as it should, actually bad with two varieties. I changed scopes not really a change.
Lower and upper with different barrel are excellent performers.

What I’m trying to figure out is if it’s worth finding a different upper that isn’t so tight or if maybe the bolt/headspace was off or if the barrel is actually the culprit. There is one more ammo variety I could try but at this point the barrel has been pulled so I would need another upper.

 

[doverpack12]

Messed up and thought I was in the firearms forum. If a mod sees this and could move that would be appreciated

 

[flyfisher117]

Generally a tighter reciever/extension fit will lead to better accuracy. That's why guys get giddy about thermal fit uppers. Others will bed their barrels into the upper.

Personally I have seen little to no discernable difference moving a quality barrel from a sloppy Aero reciever to a thermal fit BCM reciever.

 

[Eric_F]

A tight fit has been shown to be better, if you're having issues I'd look elsewhere.

 

[Trout&Buck]

Not sure if you have already checked this but if you are using a free float handguard it wouldn't hurt to make sure that it isn't contacting any part of the barrel or gas system.

 

[RockAndSage]

the barrel extension got extremely tight in the upper for the last 1/4-1/8” to the point I had to put the barrel nut on and use that to seat the barrel then backed off checked and torqued

This concerns me, as it indicates a lack of true in the bore of the upper, with the inner diameter walls not genuinely being parallel all the way, and also likely not being uniform in how it tapers. This is not the same as an intentionally-designed thermal fit. Those have more or less properly true, parallel sidewalls.

What that can easily result in, is your barrel not fitting precisely flush against the receiver - leading to the barrel's bore being misaligned to the bolt.

That leads to bolt lugs not mating flush against the locking lugs on the inside of the barrel extension, with one side being higher or lower.

That leads to inconsistencies in how the cartridge sits in the chamber both before and during firing, and what the bullet is doing when the force of firing is first applied to it. This misalignment can cause the bullet to move forward without its axis being parallel and true to the bore axis - leading to less consistent flight.

The first thing I'd do is pull the barrel and properly lap the upper. Brownell's sells a kit for it that's around $45, IIRC, and it's fairly simple. Plenty of videos on YouTube that cover how to do it right, as well.

 

[doverpack12]

This concerns me, as it indicates a lack of true in the bore of the upper, with the inner diameter walls not genuinely being parallel all the way, and also likely not being uniform in how it tapers. This is not the same as an intentionally-designed thermal fit. Those have more or less properly true, parallel sidewalls.

What that can easily result in, is your barrel not fitting precisely flush against the receiver - leading to the barrel's bore being misaligned to the bolt.

That leads to bolt lugs not mating flush against the locking lugs on the inside of the barrel extension, with one side being higher or lower.

That leads to inconsistencies in how the cartridge sits in the chamber both before and during firing, and what the bullet is doing when the force of firing is first applied to it. This misalignment can cause the bullet to move forward without its axis being parallel and true to the bore axis - leading to less consistent flight.

The first thing I'd do is pull the barrel and properly lap the upper. Brownell's sells a kit for it that's around $45, IIRC, and it's fairly simple. Plenty of videos on YouTube that cover how to do it right, as well.

Does the upper shooting well with a different barrel both before and after the barrel in question eliminate this? I’m all for lapping if it makes the barrel shoot.

 

[RockAndSage]

Does the upper shooting well with a different barrel both before and after the barrel in question eliminate this? I’m all for lapping if it makes the barrel shoot.

That's a good question, and it would indicate the problem is the barrel, not the upper. Lapping is still a good idea when doing a build, but if it shoots great with a different barrel, including after trying to install this expensive one, it doesn't sound like the upper's the problem. Expensive barrels, and expensive rifles, can and do occasionally just get misassembled, or something isn't recognized as out of spec by otherwise competent companies. It's a big bummer when it happens (and people have a hard time believing you when you're the one experiencing it), but it just doesn't sound like the problem is the upper.

 

[LaHunter]

What barrel manufacture and who chambered it? How much effort was involved in torquing down the barrel nut to fully seat the barrel extension into the upper? That possibly may have done some damage. A tight thermal fit can be a good thing for accuracy / precision. As mentioned above, what handgard do you have? That can impact accuracy also.