barrel too tight for upp

Melnic · Jun 8, 2020

barrel too tight for upper

I have 2 spare uppers and a new Rainnier Arms barrel. well, the barrel won't insert all the way in either one. I have 3 various other barrels I pulled from other uppers and all 3 of those fit. I've assembled probably a dozen uppers for me or other friends and not yet so far had this problem on any of them.

Sent an email to Rainner and got the auto reply back that they are very busy.

Any suggestions?

SkiPatrolDude · Jun 8, 2020

heat up the upper, and cool down the barrel.

I put my upper in a toaster at 250 for 10 minutes and my barrel in the freezer and it slid right in (Criterion barrel into BCM upper).

Or you can use a heat gun, hair drier, whatever. Use some Aeroshell around the barrel extension and be prepared with a rubber or nylon mallet to tap it home. If you must tap the crown of the barrel, make sure there's something soft between your mallet and the barrel.

atblis · Jun 8, 2020

This is likely a good thing. Take a hair dryer, and heat the upper. Barrel will likely slide right in. Aluminum has a fairly high coefficient of thermal expansion. Can also use a propane torch or heat gun (judiciously). I've never found putting the barrel in the freezer necessary, but it may help and others report have success. The heat gun has always gotten it done for me.

ETA: If the barrel goes part way in and then get stuck (for instance if the upper cools), a little heat on the side of the upper will get it back apart.

DutchV · Jun 8, 2020

Thermal fit is a good thing. Not a surprise from Rainier. A little heat won't hurt the upper.

ETA: Here's a video from JP about thermal fit uppers.

https://www.youtube.com/watch?v=-cOlNGxUhsk

atblis · Jun 8, 2020

The newer BCM uppers are intentionally made a little tight for this very reason.

outrider58 · Jun 8, 2020

atblis said:
The newer BCM uppers are intentionally made a little tight for this very reason.

^^^This

SkiPatrolDude · Jun 8, 2020

outrider58 said:
^^^This

Not sure OP has a BCM Upper. He also mentioned several other barrels fit np. Ranier arms barrel might just be a tad on the large side.

Not a bad thing - thermal fit is where it's at!

3paul10 · Jun 8, 2020

They are all correct. Thermal fit.

John from MD · Jun 8, 2020

Here is my .02. AR barrel to receiver fit was meant to be a press fit. What you guys are talking about is basically a shrink fit. The problem with a shrink fit in this instance is the difference in material. With a shrink fit, the steel barrel extension is putting pressure on an aluminum receiver. When the barrel heats up, it puts more stress on the receiver.

While I have used a shrink fit on other rifles, they have always been steel on steel.

My opinion is worth what you paid for it.

atblis · Jun 8, 2020

If the barrel heats up, wouldn't the upper receiver portion directly contacting the barrel also heat up to a similar degree? My guess is that as the barrel and upper both heat up, the stress actually goes down due to the receiver expanding more than the barrel.
CTE of 7075 is ~13 µin/in-°F
CTE of 4150 is ~7 µin/in-°F
CTE of 416 R is ~6 µin/in-°F

John from MD · Jun 8, 2020

But, is the receiver supposed to heat up that much?

outrider58 · Jun 8, 2020

SkiPatrolDude said:
Not sure OP has a BCM Upper. He also mentioned several other barrels fit np. Ranier arms barrel might just be a tad on the large side.

Not a bad thing - thermal fit is where it's at!

It's not just BCM uppers that are tight.

I have never had a problem fitting Rainier barrels. I wonder if Melnic trued the upper receiver face. Sometimes, just the turning of the tool inside the upper extension is enough to loosen things up.

outrider58 · Jun 8, 2020

John from MD said:
But, is the receiver supposed to heat up that much?

It's gonna get a lot hotter from firing than a hair drier or heat gun will get it, I would think.

Melnic · Jun 8, 2020

Thanks all, Heat gun to the front of the upper did the trick.
I"m sure if I were more experienced i'd have known that. Well, I know it now.
Upper is BAFE (square forge mark)

Upper assembly is all built up and ready to go.

3paul10 · Jun 8, 2020

Nice job bro.

teratos · Jun 8, 2020

Melnic said:
Thanks all, Heat gun to the front of the upper did the trick.
I"m sure if I were more experienced i'd have known that. Well, I know it now.
Upper is BAFE (square forge mark)

Upper assembly is all built up and ready to go.

Now you are more experienced...I broke more than one BCM upper trying to true it. I learned the hard way.

atblis · Jun 9, 2020

Works on gas blocks too BTW.

DutchV · Jun 9, 2020

atblis said:
Works on gas blocks too BTW.

It is also acceptable to hone the I.D. of a gas block if it is too tight.

barrel too tight for upp

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