Just a thought...… have you tried removing the barrel from the rifle and inserting a pellet in the breech end and manually pushing it through the barrel in order to see if you can feel any tight spots anywhere along the length of the barrel?
Be careful with what you use to push the pellet through the barrel. Something firm but relatively soft compared to the steel of the barrel (maybe a brass or aluminium rod) should do.
There could be a tightening of the barrel at the muzzle end, called a choke, and it is to be expected.
Examine your pellet for land marks, grooves left on the pellet by your barrel. Hopefully this would eliminate that your barrel is not causing any damage to your pellets and causing your grouping issues.
Hopefully your gunsmith will return your rifle to you in full working order.