Gunsmithing Upper Rail Missalignment with Barrel

[FromMyColdDeadHand]

I posted this in the scope section, but didn't get any feed back, so I thought this might be a good place to post it.

I have an LMT MWS. The 16 inch barrel it came with was easy to zero. I have a Premier 3-15 in a Larue mount that I have shot on other ARs so I plopped it on and I was on a 8x11 sheet target- easy to zero in. I bought a 18inch SS barrel and its POI was about 2.5 feet to the left and up. I got it zero'd in at 100 but haven't shot it farther than that.
I've seated and unseated the barrel a few times because I thought maybe it was seated wrong, and the POI changes with-in 1 MOA- that is a tight lock-up system!

I know that the barrel and upper reciever are never perfectly aligned, but at what point does the barrel and the top rail being in different directions affect scopes ability to compensate? It seems to me the up and down is not much of an issue- but I'm picturing the barrel pointed straight ahead and the scopes axis is pointed to the right- am I going to have to make windage adjustments at longer ranges to compensate. How true to each other does the rail and barrel have to be?

Thanks!

 

[Wheres-Waldo]

Re: Upper Rail Missalignment with Barrel

If your barrel/optic is not aligned with reguards to windage, you will see exponential deflection as the range extends.

 

[FromMyColdDeadHand]

Re: Upper Rail Missalignment with Barrel

Thanks. Is there a way to measure the deviation and what is considered too much?

 

[bptactical]

Re: Upper Rail Missalignment with Barrel

I would be suspect of the front ring of the upper receiver and its true relative to the rest of the upper.
If this is a new rifle I would contact LMT.

 

[Oakland Tactical]

Re: Upper Rail Missalignment with Barrel

Can you measure how far the barrel is from the right and left side of the upper? you should be able to measure with a caliper and see a difference if its off. If it is not off much it may be how the mount attaches to the rail.

 

[Clark]

Re: Upper Rail Missalignment with Barrel

I put one of these Leupold magnetic bore sighters on the muzzle:
http://www.amazon.com/Leupold-Magnetic-Illuminated-Boresighter-Md/dp/B0014VRPPW
I take off the top half of the scope rings.
I turn the scope and see the cross hairs rotate.
I adjust the elevation and windage knobs until the reticle is in the center of the scope tube and rotating the scope keep the crosshairs on the same point.
I can then read the direction and magnitude of the misalignment of of the barrel and the scope rail.
Single piece mounts are easy to fix with shims and epoxy between the mount and the receiver.
Two piece mounts are hard, as the mounts must be parallel and co planar so no ring lapping will be needed.

Some things don't make any sense, until you understand them, and then they are obvious.