Idea for bedding barrel extensions

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grampajack

grampajack

AR Junkie
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I was searching for ways to tighten up a loose flange, and one guy suggested using paint to build up the barrel extension. He didn't really get any feedback, but the idea has some merits I think.

Basically what was proposed is to tape off the parts you don't want built up (the shoulder and index pin), then spin the barrel at a constant rate, perhaps in a lath at low speed, while spraying paint onto the extension. I think this is better than building up epoxy and turning on a lathe because it would build up the entire length of the extension, whereas with the epoxy you're left with a few mm of unbedded extension since the index pin gets in the way of the cutter.

It also has the advantage of being removable. There are paints that are heat resistant enough to work, yet are still relatively easy to remove with solvents, and that is certainly not the case for any epoxy that I know of.

At the very least, this seems like a better idea than loctite. The loctite will melt unless you're very careful about your rate of fire, plus it's a huge hassle to remove the barrel if you've bedded it with loctite.

So, any obvious problems, or is this worth a try?
 
Interesting idea, ive never had an extension loose enough to make me want to try something like this....the idea does have some merit tho if your short on tools and cash, especially if you used one of the heat cure ceramic paints which should be harder to compress than standard spray paint. You could build a jig to hold the barrel and spin it with a hand drill, or use a drill press if you have a variable speed drill press. If i can find a reason i might try this just to see how it works.

The issues i can see of hand are spray paints dont deliver constant volumes, but they are pretty close.
Keeping your work piece turning at the correct speed while applying the paint evenly, mechanizing the operation should help with that.
MOST OTC paint is either pretty soft, and/or not heat tolerant, use better paint.

My major question tho, is HOW out of spec are we talking on parts. For something like this id think it would have to be pretty large, and if its that large perhaps a closer look at WHY its out of spec is worth the time.
 
LoonWulf said:
Interesting idea, ive never had an extension loose enough to make me want to try something like this....the idea does have some merit tho if your short on tools and cash, especially if you used one of the heat cure ceramic paints which should be harder to compress than standard spray paint. You could build a jig to hold the barrel and spin it with a hand drill, or use a drill press if you have a variable speed drill press. If i can find a reason i might try this just to see how it works.

The issues i can see of hand are spray paints dont deliver constant volumes, but they are pretty close.
Keeping your work piece turning at the correct speed while applying the paint evenly, mechanizing the operation should help with that.
MOST OTC paint is either pretty soft, and/or not heat tolerant, use better paint.

My major question tho, is HOW out of spec are we talking on parts. For something like this id think it would have to be pretty large, and if its that large perhaps a closer look at WHY its out of spec is worth the time.
Click to expand...

If I don't have to heat the flange, and maybe even put the barrel in the freezer, then I'm not happy with it. If the barrel just slips in by hand without any persuasion then it's too loose in my opinion. So nothing would necessarily have to be out of spec for the build to benefit from this. In fact, I would go as far as to say that what I'm happy with is by definition out of spec.

ETA: I was thinking an airbrush set to a light mist would give you a very even coat. It doesn't really have to be perfect. As long as the lockup is tight, then it should be repeatable. Having everything perfectly true would be ideal, but it would be far superior to loctite. The only way to do it perfectly would be to custom fit an oversized extension, which would necessitate making your own barrel. Don't get me wrong, I would love to get into that someday, but right now I just don't have the time and energy to learn all that.
 
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grampajack said:
If I don't have to heat the flange, and maybe even put the barrel in the freezer, then I'm not happy with it. If the barrel just slips in by hand without any persuasion then it's too loose in my opinion. So nothing would necessarily have to be out of spec for the build to benefit from this. In fact, I would go as far as to say that what I'm happy with is by definition out of spec.
Click to expand...
ahh ok gotcha!
That then brings up the abrasion resistance of the paints...or lets call them coatings, to be used. Even the harder coatings might not be up to the task, BUT they should at least be more resistant to scraping than other options. You could bevel the entry to help keep it from biting into the coating as it was pressed in, possibly.
 
LoonWulf said:
ahh ok gotcha!
That then brings up the abrasion resistance of the paints...or lets call them coatings, to be used. Even the harder coatings might not be up to the task, BUT they should at least be more resistant to scraping than other options. You could bevel the entry to help keep it from biting into the coating as it was pressed in, possibly.
Click to expand...

I think cerakote or duracoat would be plenty tough in all departments, but there's no way to remove them that I know of. Which isn't a bad thing necessarily, although it would pretty much mate that barrel to that flange for life. It would also be a disaster if you accidentally got the paint too thick. And since it's permanent, it would only marginally improve upon the standard epoxy glue in method. Sure you could rebarrel the same receiver, but with precision barrels being 500 and receivers being 100 or less...well, you get the idea. I would almost rather just glue it in, then discard the upper with the barrel once it's shot out.

But even standard krylon or aervoe I think would be far more heat, abrasion, and compression resistant than loctite. You might be surprised how tough the matte finish spray paints are once you bake them on. I had an AR where someone didn't do the anodizing right and it came off in gun solvent, so I just took a can of flat black krylon to it to make it look decent again, and when I put the parts back together they did lock up really tight. Especially where the handguard slid over the barrel nut. So I can say from experience that even cheap spray paint can have a really dramatic effect on how tight things lock up.
 
Ive worked alot with spray paint and generally had good results, but for something like this i dunno....the ar with the color comming off was probably not the ano itself but the dying process not being done properly.
Also my experience with a factory Cerakoted gun, and guns ive dura(1) and cerama coated (a few) leads me to agree that if handled properly it should work. But even the factory Cerakoted gun has nicks and dings in its finish where its been banged around, which is what gives me a little doubt.
I do think the idea is worth a try tho.

Ive removed Duracoat with a sharp knife then a wire wheel, and a resand blast.
ive also carefully trimmed Ceramacoat with a razor and flaked it off. Never tried with Cerakote but it does seem harder than the other two, which might make it preferable.
 
I wouldn't trust a "paint" to be uniform enough, nor durable enough to serve as a shim. Cerakote goes on incredibly thin, wanna say 1-2thou each coat, so you'd need a lot of coats, which then could get you into the same uniformity issues as any paint.

I personally use stainless steel shim stock around the barrel extension if I ever end up with something undersized. If I can't get a 1 thou steel shim around the extension, it doesn't need shimmed.

When I really want them to be solid, I bed and fix the extension to the upper. It doesn't matter much when you're talking about $50 forged uppers tied to $300-500 barrels.

Personally, I wouldn't waste my time thinking about a coating or paint to "shim" by extensions. I think that would be a bandaid on an axe wound at best - your mind might feel better about it (mine would not), but I'd be willing to bet it doesn't actually improve long term fit.
 
Cerakote might work...best to use a thick coat then polish or lightly sand to fit. If you have a good gun smith and willing to pay the price he might be able to unpin and replace the barrel extension with a thicker new one and fit the headspace...I would not do that yourself. Green locktite would probably be able to withstand the temps if you are thinking of going that route. Not sure how hard that would be to take off in the future..
I wonder if its possible to use JB weld or an actual bedding compound on the extension for a tighter fit...I guess that idea would really depend on how loose it actually is.
 
Klint Beastwood said:
Cerakote might work...best to use a thick coat then polish or lightly sand to fit. If you have a good gun smith and willing to pay the price he might be able to unpin and replace the barrel extension with a thicker new one and fit the headspace...I would not do that yourself. Green locktite would probably be able to withstand the temps if you are thinking of going that route. Not sure how hard that would be to take off in the future..
I wonder if its possible to use JB weld or an actual bedding compound on the extension for a tighter fit...I guess that idea would really depend on how loose it actually is.
Click to expand...

That's the traditional method, to actually glue the barrel into the upper with high temp epoxy. It's super easy to do, but it isn't reversible, so you wouldn't want to do it to a really nice upper.
 
That's the traditional method, to actually glue the barrel into the upper with high temp epoxy. It's super easy to do, but it isn't reversible, so you wouldn't want to do it to a really nice upper.
Click to expand...
Which item has the problem - the barrel extension or the receiver? If one or the other is sloppy it wouldn't be a really nice upper. Just saying...
 
Klint Beastwood said:
I wonder if its possible to use JB weld or an actual bedding compound on the extension for a tighter fit...I guess that idea would really depend on how loose it actually is.
Click to expand...

People have been "gluing" barrels into extensions for almost as long as AR's have been around. Lots of options, some better than others, some easier than others.
 
Varminterror said:
People have been "gluing" barrels into extensions for almost as long as AR's have been around. Lots of options, some better than others, some easier than others.
Click to expand...
So you're saying you don't know if anyone has done the JB bedding compound, but its possible?
 
pert near said:
Which item has the problem - the barrel extension or the receiver? If one or the other is sloppy it wouldn't be a really nice upper. Just saying...
Click to expand...
I agree, people obviously wont agree, but the best way for me is to have a thicker barrel extension and lap the receiver a little and fit to work. Or just get a barrel that fits well in the receiver..i've gotten luck in the last 4 gas guns i've built.
 
Klint Beastwood said:
So you're saying you don't know if anyone has done the JB bedding compound, but its possible?
Click to expand...

No. I am saying I know people have used JB weld, and any number of other common 2 part epoxies for many, many years. I prefer to use steel putty Devcon myself, to give a more durable backbone to the bedding material. When I have enough clearance for a shim, I use devcon liquid, thickened slightly, under and over the shim to bond.

I wouldn't waste my time with a sprayed "shim."
 
Am I right to assume you are truing your receiver mortises, lapping your barrel nuts, and lapping your lugs also? Else I wouldn't spend too much time worrying about an extra thousandths or two of extension clearance.
 
Varminterror said:
No. I am saying I know people have used JB weld, and any number of other common 2 part epoxies for many, many years. I prefer to use steel putty Devcon myself, to give a more durable backbone to the bedding material. When I have enough clearance for a shim, I use devcon liquid, thickened slightly, under and over the shim to bond.

I wouldn't waste my time with a sprayed "shim."
Click to expand...

Ah yes the steel putty, I forgot about that! I though JB was more solid then the steel putty however, or maybe my memory is just incorrect.

as to the second I assume was to me,

yep you got it, big emphasis for me was to square the face and increase contact all around for inconsistent tolerance in the coating. Not really a huge deal. That was a one time deal, the rest of my gas guns, I didn't feel the need to do anything with. My last one I had a spare 80% AR10 that i decided to make a plinker out of...threw a m5 aero on top and a premium ballistic advantage barrel didn't need to do anything besides mesh the upper and lower to fit with a dremel and a whole lot of sand paper, coat, done. shoots holes through holes with factory ammo.
 
Another option I didn't think about was bedding it like you would a stock, covering the flange in release agent, then using an epoxy. That would give you a dead nuts perfect fit, be somewhat reversible, and not ruin the upper. Has anyone tried it?

Varminterror said:
Am I right to assume you are truing your receiver mortises, lapping your barrel nuts, and lapping your lugs also? Else I wouldn't spend too much time worrying about an extra thousandths or two of extension clearance.
Click to expand...

I wouldn't say a waste of time. Even with an in spec extension and flange you can still end up super loose, and I've seen quite a bit of evidence over the years that a sloppy fit badly impacts accuracy. Lapping and such I'm not sure about.
 
Generally, if I'm gluing one in, I don't worry about a release agent. I shoot forged lowers, it's a $300-500 barrel stuck to a $50 piece of aluminum. I usually replace the gas block with new barrels too, so stack another $45-110 on top, plus the gas tube, somewhere between $10 and $100 for the muzzle device, which I typically don't reuse on a shot out barrel... About the only thing I recover on a burned out barrel AR is the handguard. Not a big deal to sacrifice a $50 forging and $15 upper kit.

If the barrel nut isn't making consistent contact on the barrel extension, it'll induce warp. If the bolt lugs aren't contacting the extension evenly, you'll risk breakage. I didn't say it was a waste of time - I said I wouldn't spend time worrying about PAINTING an ultra thin shim into the works. Painting a shim only makes sense to me if the fit were too tight to use a steel shim - and if it's THAT tight, I don't bother with a shim at all. And again, when I want my uppers & barrels properly mated, I glue them together and take out all question. If it's a thick enough gap, a steel shim is a much better option, if it's a thin gap, then it's not "loose" or "sloppy." I've slipped together a lot of AR's which would hold 1/2-3/4" 10 shot groups at 100yrds, no heat or freezer or mallet required - it should be tight enough to eliminate play, but I've never seen an AR with a watertight interference fit shoot considerably better than one which only requires mild coaxing - and gluing either in eliminates any concern of slop either way.
 
Instead of going through all this trouble of painting and lathing the paint after it dried, I'd simply get a BCM upper. It's made to give a tight fit to the barrel extension.

If I did need to tighten up the fit of the barrel extension to the upper receiver, I'd paint the barrel extension with the two part epoxy paint used to prime aircraft structures and install the barrel wet. It may make it difficult to replace the barrel, but it will get the job done. That two part epoxy paint is tough.

Anodizing forms a layer of aluminum oxide on aluminum. It cannot be removed with any kind of gun solvent. However, solvent might remove the dye or paint that was applied to the Anodizing.
 
Varminterror said:
Generally, if I'm gluing one in, I don't worry about a release agent. I shoot forged lowers, it's a $300-500 barrel stuck to a $50 piece of aluminum. I usually replace the gas block with new barrels too, so stack another $45-110 on top, plus the gas tube, somewhere between $10 and $100 for the muzzle device, which I typically don't reuse on a shot out barrel... About the only thing I recover on a burned out barrel AR is the handguard. Not a big deal to sacrifice a $50 forging and $15 upper kit.

If the barrel nut isn't making consistent contact on the barrel extension, it'll induce warp. If the bolt lugs aren't contacting the extension evenly, you'll risk breakage. I didn't say it was a waste of time - I said I wouldn't spend time worrying about PAINTING an ultra thin shim into the works. Painting a shim only makes sense to me if the fit were too tight to use a steel shim - and if it's THAT tight, I don't bother with a shim at all. And again, when I want my uppers & barrels properly mated, I glue them together and take out all question. If it's a thick enough gap, a steel shim is a much better option, if it's a thin gap, then it's not "loose" or "sloppy." I've slipped together a lot of AR's which would hold 1/2-3/4" 10 shot groups at 100yrds, no heat or freezer or mallet required - it should be tight enough to eliminate play, but I've never seen an AR with a watertight interference fit shoot considerably better than one which only requires mild coaxing - and gluing either in eliminates any concern of slop either way.
Click to expand...

I don't think lapping the bolt would fix much because AR bolts, most of them at least, will always tilt due to the forces not being balanced because of the extractor. I might be prone to trying it though. Do you just use a split case like with an M14?

MistWolf said:
Instead of going through all this trouble of painting and lathing the paint after it dried, I'd simply get a BCM upper. It's made to give a tight fit to the barrel extension.

If I did need to tighten up the fit of the barrel extension to the upper receiver, I'd paint the barrel extension with the two part epoxy paint used to prime aircraft structures and install the barrel wet. It may make it difficult to replace the barrel, but it will get the job done. That two part epoxy paint is tough.

Anodizing forms a layer of aluminum oxide on aluminum. It cannot be removed with any kind of gun solvent. However, solvent might remove the dye or paint that was applied to the Anodizing.
Click to expand...

I remember that from another thread. I think it's encouraging that a company that large is going to that much trouble. Makes me think the juice is definitely worth the squeeze. I've long thought the flange was the AR's fatal flaw, so anything you can do to mitigate that situation can't be a bad thing in my opinion. Either that or spend 1800 dollars on an LMT monolithic upper.:uhoh:
 
grampajack said:
I don't think lapping the bolt would fix much because AR bolts, most of them at least, will always tilt due to the forces not being balanced because of the extractor. I might be prone to trying it though. Do you just use a split case like with an M14?
Click to expand...

Yup, same process. Split case & a spring. Marlin 336/1895 mainsprings fit nicely. Remember to seat and firmly crimp a bullet in the front end to let you knock the modified dummy out of the chamber.

You'd be amazed how little contact even high dollar bolts can have. You have 7 lugs, a bolt might tilt and rest on one or two, pinning the tail of the bolt against the bore of the BCG or the cam pin... I check all of my bolts with layout fluid upon installation, I've even had precision barrels with matched bolts come back with only spot contact on a couple lugs, and NO contact on the others. With a high volume of fire, of course, it'll work itself in, but that means I'm over stressing those "load bearing" lugs at the beginning, and my bolt position is changing over time. I'd rather spend the extra 30min and lap. Strip the bolt and go to work.

36055357512_b55a0c232a_b.jpg

Sorry - the photo sucks, but you get the idea. I didn't have a macro lens for my phone yet when that was taken.
 
Varminterror said:
Yup, same process. Split case & a spring. Marlin 336/1895 mainsprings fit nicely. Remember to seat and firmly crimp a bullet in the front end to let you knock the modified dummy out of the chamber.

You'd be amazed how little contact even high dollar bolts can have. You have 7 lugs, a bolt might tilt and rest on one or two, pinning the tail of the bolt against the bore of the BCG or the cam pin... I check all of my bolts with layout fluid upon installation, I've even had precision barrels with matched bolts come back with only spot contact on a couple lugs, and NO contact on the others. With a high volume of fire, of course, it'll work itself in, but that means I'm over stressing those "load bearing" lugs at the beginning, and my bolt position is changing over time. I'd rather spend the extra 30min and lap. Strip the bolt and go to work.

View attachment 758524

Sorry - the photo sucks, but you get the idea. I didn't have a macro lens for my phone yet when that was taken.
Click to expand...

Ever run into any problems with headspacing afterwards?
 
Not with any bolt which wasn't already on the edge of acceptance - which I would have pitched on initial test fit before I ever got to the lapping stage. You're not the bolt moving much - I've not measured the lug thicknesses before and after, but it's not removing bulk material like what turning the bolt to true the lugs might.
 
So I tried the epoxy with release agent and it worked like a charm. I had a cheap upper with lots of slop, and it fixed the problem. I almost didn't think it was going to come back out, but after applying the heat gun a little it knocked right out. The release agent was just causing it to stick a little.

I'm thinking this could even be combined with the idea to paint it. The paint would take up that last little bit of space that's left over from the release agent, bringing it to a very slightly oversized fit, which is where I want to be. Right now it's pretty solid, but it will still slide in and out by hand at room temp, which means it's going to be loosy goosy once that aluminum starts expanding.
 
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