Mag fed loads not grouping well

[VASCAR2]

Check the front edge of your mag to see if the case mouth is making contact when loading.

Awhile back this was a problem with some asc mags. If it's contacting their it could be jacking up your neck tension and allowing the bullets to change seating depth.

That was a concern and why I asked which magazines were used. Good to know the receiver was lapped and you bedded the barrel. There have been reports of muzzle devices affecting accuracy especially if a lot of torque was used to time. Many forum members prefer to use a lock nut or shims instead of crush washer.

I think if it was a muzzle device causing accuracy it would be apparent at shorter distances.

If the Standard Deviation goes up with magazine fed ammo that makes me believe the bullets are moving in the mag or during feeding.

Another possible culprit could be the sizing die if neck tension is erratic or is over working the case mouth and neck. What sizing die are you using?