View Full Version : Question on barrel fit into Upper.
settingsuns
04-09-23, 15:37
Folks, I think I already have an idea what the answer is but wanted to get some opinions from those more experienced and better informed than myself. Am I correct in believing that a tighter fit between barrel and upper receiver is conducive to better accuracy? If I have an upper that I have a fairly difficult time sliding a barrel into, as opposed to another upper that is an easy slip fit with the same barrel, then the upper with the tighter fit should provide better and more consistent accuracy? I'm asking because I have a "no-name' generic, flat top upper (Anchor Hocking foundry mark) with forward assist, that is a super tight fit with the Faxon barrel I want to use. However, I have a "name branded" flat top, slickside upper that is a noticeably looser fit with the same barrel.
I'd love to use the slickside upper but if it's more likely I'll get better and more consistent accuracy with the generic upper then I'll go that route. What do you folks think? I look forward to everyone's opinions.
It would have to be a terribly loose fit for you to see a different.
There is definitely a difference in consistency between a tight barrel to upper fit vs a loose one. You can still use your loose fitting upper with the barrel if you want to though. You can bed the barrel to the upper using Loctite or stainless steel shim stock to make it a press fit. There are videos online demonstrating this.
I have bedded two barrels this way. I bought some hardened stainless shim stock from Amazon in 0.001" thickness. One of my barrels extension to upper receivers had a 0.002" difference, so I wrapped a piece of the shim stock once around the extension, and after freezing the barrel and heating the upper, was able to mate them. The other had a 0.003" difference between the two, so, instead of buying 0.0015" shim stock as I probably should have, I simply wrapped the shim stock one and half times around the barrel extension before freezing the barrel and heating the receiver.
Both have been fine, and though I didn't notice an increase in precision in the one upper I had already fired, I did notice a decrease in stringing when hot.
The way the barrel is flanged and squeezed between the nut and the upper, you’d have to have some serious slop to notice a difference.
The way the barrel is flanged and squeezed between the nut and the upper, you’d have to have some serious slop to notice a difference.
Dunno, but a lot of the guys doing national match competitions seem to care about this, as do BCM and SOLGW. Thermal expansion seems to have a way of moving things around.
settingsuns
04-09-23, 18:31
Folks, I appreciate everyone's input. I sometimes overthink things. Thanks for bringing me back to reality. I want the best accuracy possible, but who am I kidding? it's not a benchrest rifle. Thanks again.
It would have to be a terribly loose fit for you to see a different.
This... And I'd be more worried about using a Faxon Barrel.
Barrel extension outside diameter: 0.9979 to 0.9987 inch in diameter.
Upper receiver bore diameter: 1.000 to 1.002 inch in diameter.
So, even when things are fully inside the specifications, you get anywhere from snug 0.0013" slight press fit, to a 0.0041" loose fit for the barrel in the upper.
When the barrel nut is tightened, the threads push against each other. The aluminum receiver will squish down before the steel barrel nut will expand.
When the barrel nut is tightened, the threads push against each other. The aluminum receiver will squish down before the steel barrel nut will expand.
So there is an inward clamping force as a result of the thread angle?
So there is an inward clamping force as a result of the thread angle?
Yes, there is about 4500-5000 pounds of force pushing the barrel back into the threads, so there is a equal amount trying to wedge the threads out of the way. The thread angle is 60 degrees, so the wedge angle in half that, or 30 degrees. You can calculate the constriction achieved from those numbers.
But, I can tell you you are not going to make up anywhere near 0.004" inch, 0.0001 inch maybe . . . at maximum torque.
I wonder if slots cut through the threaded ring like a collet to allow it to compress and grip the barrel would have any effect on accuracy. I won't be trying it.
I wonder if slots cut through the threaded ring like a collet to allow it to compress and grip the barrel would have any effect on accuracy. I won't be trying it.
No, it would crack before anything.
Besides, there is no coning on the inside of the barrel flange to force inward.
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