Should I buy a thermal fit upper or roll with what I have?

KCode · Jul 8, 2024

Well I got a Rosco K9 16" barrel and want to build as accurate setup as I can. I have 4 stripped uppers and I test fit them all to this barrel and they all went on the extension snugly but none were hard to put it in.

My question is for $79, should I get a thermal upper to start with? The other option is to possibly shim these. I don't have shim or have ever done it. So regardless, cost will be there.

simonp · Jul 8, 2024

If it was me and I was going for maximum accuracy I would spend the extra money for thermal fit.

KCode · Jul 8, 2024

simonp said:
If it was me and I was going for maximum accuracy I would spend the extra money for thermal fit.

That's the only real point of this build. As much accuracy as I can get out of this barrel.

Cfshooter · Jul 8, 2024

I would just glue or shim it in one of the uppers you already have. I do not think you will see the difference on target.

srt-4_uk · Jul 8, 2024

Cfshooter said:
I would just glue or shim it in one of the uppers you already have.

This right here. Shim stock and loctite are cheap and will do the same thing.

KCode · Jul 9, 2024

If you loctite the extension are you able to remove the barrel later?

Cfshooter · Jul 9, 2024

KCode said:
If you loctite the extension are you able to remove the barrel later?

Yep, pretty easy too.

Remove Glued in Barrel

Rocketvapor · Jul 9, 2024

ACCURATE measurements will tell you what your FIT really is.
Do you have a TAPER in the O.D. of the extension?
Barrel goes into the upper, easy at first, but the last little bit is tight?
That makes the fit loose in the strong area of the upper, and tight in the thin threaded area.
I think some of this taper (if not intentional) is the result of thread expansion when the extension is installed on the barrel.

So, got taper?

346ci · Jul 9, 2024

For a rosco bbl, a anderson or psa will be fine. When you want to try a better barrel, get a better upper then.

MSTN · Jul 9, 2024

Add 50 FT-LB of torque to the barrel nut and it won't be loose any more.

KYAggie · Jul 9, 2024

I didn’t see that anyone has mentioned it yet, but if you are going to the trouble to shim/loctite it you should probably true the receiver first.

In all honesty, you could probably just do nothing and it would shoot well within any needs within the rounds actual effective range. If you are punching paper or steel long range, that’s different.

AleksanderSuave · Jul 9, 2024

For what its worth, thermal fit doesn't mean you will have uniform mating between the barrel extension and the receiver. It just means its undersized.

green loctite will solve that problem. SS shim more so if you need it. And once you buy both of those, you'll likely have enough to do 50+ rifles.

KCode · Jul 9, 2024

346ci said:
For a rosco bbl, a anderson or psa will be fine. When you want to try a better barrel, get a better upper then.

That's a shit thing to say.

hlee · Jul 10, 2024

My most precise AR (boringly sub moa) is built on an Anderson upper. Take that for what it is worth…

Rocketvapor · Jul 10, 2024

I keep referring to this image for barrel stability in an upper.
Clamping an extension to a faced upper holds the barrel to the thin threaded nose nice and tight.
If the extension has taper and fits loose inside the upper back where the upper has more strength
then expect it to move, wobble, when fired.

I'm guessing extensions bulge a little in the threaded area when torqued onto the barrel.
Nice tight fit for last 1/4" just means the extension doesn't fit the upper.

Blue goo, or my Spray On shim will fill the uneven gap but how do you make a tapered shim?

Cascade Hemi · Jul 11, 2024

Rocketvapor said:
I keep referring to this image for barrel stability in an upper.
Clamping an extension to a faced upper holds the barrel to the thin threaded nose nice and tight.
If the extension has taper and fits loose inside the upper back where the upper has more strength
then expect it to move, wobble, when fired.

I'm guessing extensions bulge a little in the threaded area when torqued onto the barrel.
Nice tight fit for last 1/4" just means the extension doesn't fit the upper.

Blue goo, or my Spray On shim will fill the uneven gap but how do you make a tapered shim?
View attachment 8456419

If you get the shimmed extension in far enough for the barrel nut to catch a thread, you can apply 620 or Indian Head to the inside of the receiver, and hand thread the barrel in. The barrel will be shimmed at the snout and glued for the taper.

AleksanderSuave · Jul 11, 2024

Cascade Hemi said:
If you get the shimmed extension in far enough for the barrel nut to catch a thread, you can apply 620 or Indian Head to the inside of the receiver, and hand thread the barrel in. The barrel will be shimmed at the snout and glued for the taper.

I did exactly that at the recommendation of proof (indian head gasket shellac) with a barrel I got that could fall out on its own if the receiver was tipped barrel end down. In hindsight, I should have just exchanged the bad example of a barrel, but availability was slim at the moment and I didnt want to wait longer.

BCP · Jul 12, 2024

KCode said:
Well I got a Rosco K9 16" barrel and want to build as accurate setup as I can.

Whatever upper feels like it fits the best is fine. Maybe put a little grease on the extension, grease on the threads for sure.

That's a cheaper barrel unless it's literally rattling around in the extension I don't think you will squeeze noticeably more accuracy out of it by thermal fit. That's one of the last things you do when building a match rifle after you've covered all the other bases (custom barrel/bolt combo, trigger, handguard, receivers etc).

M4orturnate · Jul 12, 2024

KCode said:
Well I got a Rosco K9 16" barrel and want to build as accurate setup as I can. I have 4 stripped uppers and I test fit them all to this barrel and they all went on the extension snugly but none were hard to put it in.

My question is for $79, should I get a thermal upper to start with? The other option is to possibly shim these. I don't have shim or have ever done it. So regardless, cost will be there.

Any upper you have will be sufficient for a Roscoe barrel. If you were doing a custom cut barrel I'd recommend a vltor upper or to bed it.

fedupflyer · Jul 17, 2024

Cfshooter said:
I would just glue or shim it in one of the uppers you already have. I do not think you will see the difference on target.

I done correctly you will.
A few months back I took an AR armorers course on how to accurize the AR platform.
I had an old project gun which I had apart more times than I can count and the fit was a little loose.
As part of the class we test first with match grade ammo to set a base line. I was able to take a criterion barrel from 1.5 MOA to somewhere between .55 & .8 MOA by shimming the barrel extension into the upper and using green loctite to secure it on place.
So yes, it does work.

If you are going to build an AR, why not go ahead and spend the few extra dollars and squeeze as much mechanical accuracy out of it as possible.

But, I would prefer to have a thermal fit upper and still loctite it in.

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Should I buy a thermal fit upper or roll with what I have?

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