Muzzle doesn't matter much if the bullet is driving into the lands in the throat/lead
You need to get a caliper and start measuring... the neck might be to tight.?
Your field gauge only tells you the headspace is not out of spec.
You need a go gauge to see if minimum HS is good.
Also being a replacement barrel we have no idea if the chamber was ever finished correctly... any close possibilities of another cartridge these where know to be chambered in?