Gunsmithing Is this 308 receiver to barrel fit too loose? Are my tolerances normal?

[Long Range]

I'm using an aero precision ar10 upper and a faxon 308 barrel.

The barrel slides into the receiver easily. If I put the barrel into the receiver and then hold the barrel, the receiver will easily slide off the barrel by tilting the barrel. I don't feel any resistance at all when pushing the barrel into the receiver.

The measurements using a precision caliper are:

308 barrel extension: 1.1860"
Receiver = 1.1900"

When measuring the barrel, I would say it ranged while going around the barrel from 1.1850 to 1.1870. The largest I could find the receiver hole was 1.1900"

Would anyone know if these tolerances are OK for a 308 rifle? Will this kill my accuracy?

Do you know if its normal for the receiver to so easily slide off the barrel, or should you have to use a little force to get the barrel in?

I do know I had an FDE 5.56 aero receiver and it felt tighter when installing the barrel, maybe this was because of the FDE painting? I have other 5.56 black aero receivers and the barrel slides it easily also. The only tight fit I seemed to have was with the FDE.

 

[GH41]

Your measurements probably lack accuracy. Sounds like both manufacturers erred on the opposite sides of the print. Not unusual at all in the lower end stuff. I don't believe a tight fit is worth the effort for the average build and shooter. If it makes you feel better glue it in with LocTite sleeve retainer.

 

[Long Range]

Does anyone know the tolerances on 308 barrel extension and the receiver hole it gets put into?

 

[svxwilson]

Buy some stainless steel shim stock and thermofit the barrel to the receiver. You wrap the barrel extension with a custom shim you make, super heat the receiver in an oven or with a heat gun, getting it to expand to a dimension slighly larger than originally. Then slide the barrel in while the receiver is HOT and within seconds you will have a rock hard, locked in connection. I did this to all my ARs and honestly saw improvement in all of them.

This is what some of the most reputable accurate AR builders do, if the receiver isn't already a thermofit receiver with undersized ID in the extension.

Some people go the extra step and use locktight and the shims. From my research locktight 609 will work better for the application than standard red.

 

[larryh128]

1st thing I would do is put it together & shoot it. Doesn't take much to take them apart.

 

[srt-4_uk]

Does anyone know the tolerances on 308 barrel extension and the receiver hole it gets put into?

It's in the mil spec....

 

[sirhrmechanic]

Your measurements probably lack accuracy. Sounds like both manufacturers erred on the opposite sides of the print. Not unusual at all in the lower end stuff. I don't believe a tight fit is worth the effort for the average build and shooter. If it makes you feel better glue it in with LocTite sleeve retainer.

.004” is a lot to ask for Loctite, even sleeve retainer. But with steel shim stock, a shrink fit and Loctite... the package would definitely be tight!

I‘ve never had an so loose I needed a sealer or shims, but as was pointed out, if everyone maxed out their tolerances... I have a heavy barreled varmint AR that never shot like I think it should. So it hangs on my wall. I’m going back to give the Loctite a try.

Great thread, never thought about Loctiting in an AR barrel. But it makes sense for accuracy!

Anyone ever looked at coefficient of Thermals expansion between barrel and upper? Aluminum is about 5x steel. Though I can’t imagine it matters much in the slip fit area... but is another area where stacking tolerances work against the barrel/receiver fit.

Definitely following this thread!

Cheers, Sirhr

PS. About six months ago got an AR-9 pistol from a maker that will go unnamed. It was cute and all of $359 complete on gunbroker. And I figured... meh, even if it doesn’t run, I can get it working no problem. 16 hours later... I have it running like a top. It needed everything imaginable.... from feed ramp to buffer to mag release to nfg extractor to bent ejector to barrel fit. Nothing and I mean nothing worked. I’d fix one thing and go “ha, I got you this time...” and a new problem showed up. This gun was clearly thrown together from parts, never tested and shipped out as a “you fix it.” If I add up my time, I could have bought a nice Colt 9mm and had something worthwhile out of the box. I can’t imagine how much it must suck for the dozens or hundreds of other people who bought those same POS 9mms off gunbroker and Don’t have A huge workshop or tons of machining experience and ability to make their Jam-O-Matic bargain AR run. If they took it to a local smith to fix, they would have had a $1500 labor bill on a $359 gun. AR’s are not just Revell kits!!!

 

[Hetzer]

I have the same issue. Aero M5 receiver and a Criterion barrel that seem pretty loose. Barrel just came in and I haven't measured it. Certainly not thermo fit. May not be enough for a shim. We'll see.

 

[GH41]

638 will work to .25mm. That's almost 10K. I am not so sure he has a 4k problem. He measured the ID with a caliper. Shims with loctite would be like trying to push a greasy string up a cat's ass.

 

[sirhrmechanic]

Anyone ever tried knurling the seating area of a barrel? Between that and Loctite, would definitely tighten things up. Sirhr

 

[phxfa]

I’ve used cerakote if I’m already coating the receiver. I also freeze parts to shrink fit. 1911 and SIG rear sights especially