- Thread starter Winny94
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Thermo-fitting is the better approach. Loctite is a band-aid.
That's rarely an option and in my case definitely not an option as I already have the upper and barrel
Any barrel can be thermofit to a receiver.
How so? If the extension is a loose fit, heating the upper will only make it looser and heating the extension will only temporarily tighten it up
Thermal fitting is more than just heating the upper and cooling the barrel. If the fit is loose at room temperature, then you need shim stock. Wrap the barrel extension with shim stock to a few thousandths over the ID of the upper. Then barrel in the freezer, heat upper to about 200 with a heat gun. Mate the parts, let cool and viola, thermal fit. While some manufacturers use Locktite in their thermal fitting, it is not needed at all. Most thread locking compounds (including red loctite) work by a controlled oxidation process. Since the metal of AR15 receivers are passivated, that prevents the corrosion process from working as intended and in most cases, leaves a non-solidified material for a long time. The benefits are marginally better and in some cases won't help for weeks until the material hardens, and then the zero can wander.
Ah, that's not what I consider thermal fitting, Ive always referred to that as shimming, then thermal fitting - 2 separate process. I've not had much luck sourcing the correct shimming material so that's why I was asking loctite 620, but if you have a source for the shim stock, I'd prefer to go that route
Enough to last you several lifetimes: https://www.amazon.com/PRECISION-BR...15259087&sr=1-3&keywords=sheet+stainless+shim
I made a template, cut off a strip, roll it around a dowel to form it. Thermal fit with shims is the original, and actually what companies like JP do.
I made a template, cut off a strip, roll it around a dowel to form it. Thermal fit with shims is the original, and actually what companies like JP do.
620 is specifically made for cylindrical gaps and works well for bedding barrels. Its not a band aid. It will take a torch to get the barrel out though as you need 500f to release. Heat close enough to the extension till it smokes then tap it out with a wooden dowl in the receiver.
Really?
http://www.loctite.com.au/3320_AUE_HTML.htm?nodeid=8802627584001
“Loctite® 620™ is a high temperature (450°F), high viscosity liquid retaining compound for cylindrical bonding .Provides a shear strength of over 3,800 psi on steel. Locks and secures metal cylindrical assemblies up to 0.015" diametral clearance. Prevents metal fretting and corrosion, Stops leakages. Typical applications
include locating pins in radiator assemblies, sleeves into pump housings and bearings in auto transmissions.”
You've got some misinformation there about how Loctite works. It is anaerobic, meaning it cures in the absence of oxygen, and kicks off by reacting with the surrounding metal. It may not kick off very well if you're bonding one anodized aluminum part to another, but the barrel extension is steel and provides all the reaction surface it needs. Maybe there's an issue with some of the coated barrel extensions, but that's the exception, not the rule. Same goes for mounting a free float handguard to a barrel nut; even with blued or parkerized barrel nuts, the Loctite reaction still happens as intended and cures properly.
Where you won't see it cure is the squeezed out excess around the edges, because it's exposed to oxygen. That has to be wiped off, and is no indication whether or not the stuff inside the joint cured.
Every upper I build has its Received Face Trued, use an oversized barrel extension with Red Loctite, and a tight fitting upper.
Oh, I fully understand that the cure occurs in the absence of Oxygen. Oxygen is not the only oxidizer, far from it. 
Carbon Steels are different from Stainless steels in their place in the Galvanic series. You might call Loctite at 800-624-7767 and ask them how to successfully use their products for the intended application. I have, several times, a few of those for cases of failures. They know their product line, to include the primers and activators.
Carbon Steels are different from Stainless steels in their place in the Galvanic series. You might call Loctite at 800-624-7767 and ask them how to successfully use their products for the intended application. I have, several times, a few of those for cases of failures. They know their product line, to include the primers and activators. 
Oh, I fully understand that the cure occurs in the absence of Oxygen. Oxygen is not the only oxidizer, far from it.
Are you trying to say that Loctite products do not work in contact with carbon steel barrel extensions? Really?
If not, what exactly are you trying to say there, and what is your point?Only if you're a bubba-bumpfire retard who likes to see your barrel turn purple and blue from the heat.
2 or 3 mags of steady fire will easily get your barrel up to 500 degrees....you dont need to be a "bubba bumpfire retard"( seriously are you fucking 12?)......anyone whos taken any sort of training course will do that no problem....
but if youre the type who only benchrests their gun and puts 10 round down range in 30 minutes....i suspect you wont have an issue.
I like how none of the people who answered in a negative way have actually used this on their guns, or at least mentioned using it.
The short answer is yes, 620 is good to go for bedding. As previously mentioned, it is for press fit applications with tolerance gap of .005 between mating surfaces. It’s retaining strength of around 4K lbs is more than adequate for helping reinforce the barrel extension/reciever extension mating especially in longer barrels. I’ve used it on all my builds since 2015, 11 in total ranging from precision 24” varmint rifles to 10.5” sbr’s. These see ever possible kind of shooting from slow benchrest firing to carbine classes to tactical shoots. The rapid fire heat has not been an issue I’ve noticed and the 2 I’ve had to disassemble have been pretty easy, just heat using a industrial heat gun and press out with a 1 ton hydraulic press. No damage to the extension or reciever. Hope this helps.
The short answer is yes, 620 is good to go for bedding. As previously mentioned, it is for press fit applications with tolerance gap of .005 between mating surfaces. It’s retaining strength of around 4K lbs is more than adequate for helping reinforce the barrel extension/reciever extension mating especially in longer barrels. I’ve used it on all my builds since 2015, 11 in total ranging from precision 24” varmint rifles to 10.5” sbr’s. These see ever possible kind of shooting from slow benchrest firing to carbine classes to tactical shoots. The rapid fire heat has not been an issue I’ve noticed and the 2 I’ve had to disassemble have been pretty easy, just heat using a industrial heat gun and press out with a 1 ton hydraulic press. No damage to the extension or reciever. Hope this helps.
How does 620 compare to rocksett for this purpose. I've always used rocksett since it's near the chamber and it can handle high temperatures. It's sounds like 620 is stronger.
Used it on the first 20 or so AR15s I built, then was educated on it by an armorer from the AMU. Did my own research, talked to Loctite and changed methods. Carry on.
Rocksett is fine, no red loctite isn't going to cook off either...
Never heard of using shimstock strips to fit a barrel extension, learn something new every day....
Never heard of using shimstock strips to fit a barrel extension, learn something new every day....
Mark
How about sharing with us less fortunate as to AMU protocol? Seems it could be of benefit to a larger assemblage here. Propriety has a place, and in the spirit of sharing, it isn’t here.
How about sharing with us less fortunate as to AMU protocol? Seems it could be of benefit to a larger assemblage here. Propriety has a place, and in the spirit of sharing, it isn’t here.
The other thing to question here is if your having to shim the fit, probably with .005 or .010 stock, you’ve either got an undersized barrel extension or an oversize receiver. No amount of loctite is going make up that difference in fit so yeah I see why you changed methods.
Concealed carry on.
I’m sure the Co he works for wouldn’t want him sharing their proprietary method.
Ill share my proprietary method for precision AR15 rifles. Buy 5 heavy walled 7075 uppers, using a mandrel and lathe to find internal runout of the upper extensions, find the one that has the most concentricity aka least runout. Return the rest.
Once you have found a better than average one in the .001 or under range, face the extension mating surface, or lap it if you have the tool and compound. Next using a bat machine 1.000 oversize extension, turn it down so you have a tight, >.002 press fit, much of the time I heat the receiver extension to get the barrel in. Now is the time to use that 620 on the receiver extension. That is the proper way to bed a barrel according to the sources I trust.
Just an FYI... Criterion Barrels recommends Loctite 609.
Is that one better then 620 ? I don't know.... but I have used Loctite 609 on 5 barrels so far. 3 .308 AR's , 6.5CM and 1 5.56, no issues
Is that one better then 620 ? I don't know.... but I have used Loctite 609 on 5 barrels so far. 3 .308 AR's , 6.5CM and 1 5.56, no issues
Why not just buy a BCM upper and install the barrel? They're designed for a thermofit from the start. I've started using those myself after learning that fact.
My experiences are similar, although when I use Loctite for this, I use #680; IIRC it's a little stronger than 609 or 620 and is designed for slip fits. (It's also green of course.)
I also use it for mounting handguards and bedding scope bases on bolt guns; it's a pretty good general purpose grade. Just don't use it for press fits, especially if there's any heat involved, it cures really fast in those conditions and I've had it lock parts in place before they're fully seated.
I've never had one break down the Loctite from shooting; they always take a bit of effort and heat to disassemble, which is a good thing IMO. Same goes for a thermofit upper too of course.
I have no disagreement with those saying a thermofit barrel/upper joint is best, and agree with that in fact. However, a properly done Loctited upper is pretty good too, and more than adequate for a lot of AR applications; it's certainly not a band-aid. Either method is better than just slipping the parts together as most people do.
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