The LBT measuring slug will allow you to feel any restrictions. With the LBT slugs, you can check the muzzle diameter, then run it through the bore and see if it comes out smaller. I would try this with your slugs if possible. Since you already know how it come out you only need to check the muzzle.
It sounds like firelapping will be the way to go. If there is a restriction ahead of the chamber, this will iron it out.