Barrel alignment problem?

[SpairParts]

Im building my first rifle 6.5 Grendel using a 20in MCGowen barrel and Aero Precision upper.
The receiver is flat top. If i use a digital protractor on the flat part of the upper and make it 0.0 then place it on the barrel, the barrel reads 0.2 low.
I assume this is a problem and will affect accuracy but im unsure.
Maybe the upper rail is not level with the bore of the receiver?
The receiver has been lapped with the drill style lapping tool.
Any ideas on how to proceed or how to check verify things?
Thanks for any help!

[VASCAR2]

Wild guess here but, not unheard of not getting barrel extension square with barrel. Also not unheard of having a top pic rail not square. Slight possibilty barrel has a little downward bend, or bore was not cut true and centered. I’d think this would be obvious and doubt that is the case. If it were mine and if the receiver lapped true I’d install the barrel.

I’d be surprised if it doesn’t sight in and shoot great. Hopefully some of the more knowledgeable members will chime in.

[Randy99CL]

So that 0.2 low is 2/10ths of a degree low??

On another forum I read that Ruger is having trouble boring the barrel hole square with the 10/22 receiver.
Sure enough, when I bought my last new one a few months ago I checked it with a straight edge and my barrel is not centered in the receiver. It's cocked bad enough that you can see it is off, probably 1/16."
Lots of people have found this problem and tried to get Ruger to fix it but they say the guns are within spec.

Turns out it doesn't affect the accuracy or function of the gun. The bolt still headspaces correctly and the barrel is still solid within the receiver. The only problem is that some have had trouble with scopes that don't have much windage adjustment.

[VASCAR2]

I wondered if the opening for barrel extension in the receiver might not be square with the top Picatinny rail but the the lapping of the receiver would mitigate any allignment between the bolt/carrier and the barrel extension (I think). If the barrel was angled down .2 degrees this would have the same effect as using a 10 MOA mount lowering the zero on the scope. I think the scope and rifle should zero in but the reticle would not be in the center of it’s adjustment range. It is very common for windage error because of canted front sights or receiver faces not being square.


With the barrel being McGowen I would suspect a problem with a forged upper receiver before a bad barrel. It would be interesting to put the McGowen barrel in another forged or billet receiver to see if the problem followed the barrel. My bet is it’s an out of spec upper.

[SpairParts]

Yes 2/10ths
So i tried supporting the barrel and taking a reading.
Barrel reads 0.0 and receiver reads 0.0
Im starting to think when i support the receiver at 0.0 it is not as solid as i think and moves just enough to give me a bad reading on the barrel.

[Bigs28]

Did you lap with the receiver horizontal or verticle?

[NugginFutz]

Yes 2/10ths
So i tried supporting the barrel and taking a reading.
Barrel reads 0.0 and receiver reads 0.0
Im starting to think when i support the receiver at 0.0 it is not as solid as i think and moves just enough to give me a bad reading on the barrel.

Just a shot in the dark, here, but did you actually tighten down the barrel nut before taking the reading on the barrel?

[Wafavre2]

Lock the receiver down with the barrel going vertically. Eliminates sag and gravity, use Shim stock to take most of the play out from between barrel (apply Blue Locktite) and receiver and tighten barrel nut

[Sticks]

Even if it is low, all that does is give you a little less elevation. Shoot it. If you are going for long range, you are putting a 20MOA set of rings on there to get out there, which will soak up the .2*.

[just_john]

DId you check to see if there was some taper to the barrel? If so, that could give you the same result and the bore still be straight.

[SpairParts]

Ok sorry been busy!
Yes barrel is tight. Receiver was lapped Horizontal, I did not use blue locktite. Barrel is not tapered.
Hung vertically i still measure it at 0.1 tenth deg off.

[NugginFutz]

Ok sorry been busy!
Yes barrel is tight. Receiver was lapped Horizontal, I did not use blue locktite. Barrel is not tapered.
Hung vertically i still measure it at 0.1 tenth deg off.

Well, then.

With a 1/10th difference in misalignment from horizontal to vertical, there is clearly something moving where it should be rock solid.

It sounds like it may yet require a bit more lapping. That is, unless the barrel extension is loose / deformed / or otherwise not true. I recently read a thread on another board where the OP was installing a muzzle device and spun the barrel within the threads of the extension. That would have had a similar effect as you're describing.

Does the misalignment appear when the barrel and receiver are laid on its side (rotate 90'). If so, it would suggest a loose fit in the threaded portion of the extension.

In any case, until that movement is eliminated, the rifle will not likely shoot straight under any conditions.

Perhaps someone else can pipe in with more thoughts on the matter...

[Bigs28]

Lap vertically.....

https://youtu.be/HAL5DqNJd9s

You can skip to 4:25 if you want

[Kswhitetails]

More thoughts on the matter!

[NugginFutz]

More thoughts on the matter!

Thank you, KSWT. I knew you could be counted on.