I have polished chambers using a case with primer vent drilled out and a #6 bolt inserted. The case is covered with polishing compound and spun. That is best done in a lathe but with care, can be done with care using an electric drill. However, using anything more aggressive than polishing compound could have negative results. I did that to eliminate some minor chamber roughness.

You might try running a lubed case thru a 357 die which has a head diameter of about 0.394" and then see if that seats. If it does seat in both chambers, I suggest doing with a second case & then fire form those cases, and load with 200 gr. 35 Rem bullets and see how they seat. If they seat well (they should), then your issue is resolved.