This problem is not usually an expensive correction, and I do happen to have a frame here that can be used to test-fire a 12 ga. M-11 barrel.
If you had a 16 or 20 ga., I'd need the entire gun to test pattern the point of impact for verification of corrective action.
Not too long ago I did have a 16 ga. Winchester M-12 here that had been hitting 8" left unbeknownst to the owner for some time. After the straightening of the obvious leftward curve, it was hitting straight on and just a bit high in patterns that I pictured to email to the client for his approval of results.
It is better to have specific pattern results from the shooter of the error to ensure that my testing of the error duplicates that amount, so then the correction should be in a similar impact zone once returned and re-tested by the sender in comparison to my final impact checks.
Barrel offset to right making impact to left- indicator readings listed.