Interesting Barrel and gas block problem

[Darklyte27]

The 18 inch barrel has some rub mark where the gas block was being stopped, I thought it was the set screws but backed them out. I thought maybe the gas block was the wrong size, its a sadlak ind .750, I also tried the bravo company mfg block also .750 with no go. Both gas blocks work just fine on the 16 inch barrel.
My guess when the barrel was mfg some how the rub mark part wasnt cut down far enough?

I have 3 RA barrels, a 16 inch med con first *works fine* and then the 16 heavy and 18 med con.

here are pictures

[TKM]

What does Mr. Mike say?

[SJgunguy24]

Get a brake hone and open up the gas block some.

[ar15barrels]

Get a brake hone and open up the gas block some.

Even easier it to spin the barrel in the lathe and hit it with a fine cut file.
That will take off up to a couple thousandths reliably.
If it is right on the edge of almost fitting, just heat the block up with a torch and slip it (quickly!) into place.
I had to do that with a 300blackout LMT MRP barrel today after cutting it down to 8.5" long and opening up the gas port.

[SJgunguy24]

Even easier it to spin the barrel in the lathe and hit it with a fine cut file.
That will take off up to a couple thousandths reliably.
If it is right on the edge of almost fitting, just heat the block up with a torch and slip it (quickly!) into place.
I had to do that with a 300blackout LMT MRP barrel today after cutting it down to 8.5" long and opening up the gas port.

Not everyone has a lathe, i've used a drill press to do the same. As long as I have something to back that part of the barrel up so it cuts even and won't side load the chuck too bad.
8.5" mrp in 300blk? Did the rail have to be chopped down too? Pistol or supressed SBR?

[Darklyte27]

I emailed joebob he emailed john at ra. Ra is in the middle of movin!

No rush im sure ra will take care of it.
I dont have any tools that can do that.

[ar15barrels]

8.5" mrp in 300blk? Did the rail have to be chopped down too? Pistol or supressed SBR?

Its for a guy in Texas.
He only sent the barrel.
LMT MRP is a quick-change setup.
Loosen one screw, remove another and the barrel slips out.
Slip in a new barrel, install one screw and tighten the other screw and you are back in action.

[Darklyte27]

John at ra said his barrels are cut to the exact specification of .750 so his gas blocks are actually .755. He says that the stag and some other brand gas blocks actually run a little small. The gas blocks that I have are BCM sadlak and Yankee Hill manufacture.

Ra will hone my gas block to fit this barrel.

[SJgunguy24]

I emailed joebob he emailed john at ra. Ra is in the middle of movin!

No rush im sure ra will take care of it.
I dont have any tools that can do that.

You can get a 3/4" hone from any decent auto parts store for around 20-30$. You just chuck that up into your drill and go slowly until you get the fit you like.

[Darklyte27]

I took my calipers and measured the barrel again just for the heck of it, where the block sits is .750, at the rub spot its about .7510 - .7515

I think it might really be the barrel... well see when i get the block back.

[ar15barrels]

Typical colt 3/4" barrels measure 0.7498" at the gas block seat.
Then most quality front sight bases are bored 0.7500".
They are a tight slip fit.
Most setscrew gas blocks are 0.751" bore.
PRI gas blocks run tight at 0.750" ID.
Almost every barrel I measure is anywhere from 0.0002" to 0.0005" undersized from 3/4" at the gas block seat except for Compass Lake that measure consistently at 0.7500".
I'm measuring with a proper set of 4.5 decimal place mitutoyo micrometers that I trust.
I would not trust calipers to be accurate beyond 0.002", especially in the hands of someone that does not use them daily...

[Darklyte27]

I concur

Typical colt 3/4" barrels measure 0.7498" at the gas block seat.
Then most quality front sight bases are bored 0.7500".
They are a tight slip fit.
Most setscrew gas blocks are 0.751" bore.
PRZi gas blocks run tight at 0.750" ID.
Almost every barrel I measure is anywhere from 0.0002" to 0.0005" undersized from 3/4" at the gas block seat except for Compass Lake that measure consistently at 0.7500".
I'm measuring with a proper set of 4.5 decimal place mitutoyo micrometers that I trust.
I would not trust calipers to be accurate beyond 0.002", especially on the hands of someone that does not use them daily...

[Darklyte27]

So i got a call from ra the minimum charge for gun smithing is 15$
I wish they would have told me that before i sent it off. Plus the 7.95$ shipping.

[Darklyte27]

just a FYI, block came back and it still didnt fit. RA sent me a label and shipped back barrel and block. they took care of it and it fits now.