In answer to your question OP, the usual procedure before parkerizing was and is abrasive blasting. In the early days they used sand, and pretty coarse sand too, looking at some of the UK made No4s from 1944/45. Now we use glass beads more often and the finer they are, and the lower the blasting pressure, the less etching or stippling of the surface there will be.
If your rifle is parkerized now, 90% or better chance it was done in military service. "Restoration" of the finish then becomes something of a mute point in most people's opinions.
Conversely, if your gun was blued originally you would need to determine what the original blue and polish was, and whether you would need to polish it to make it as close to the original finish as you can get. If you do have to polish, it would require a lot of care to avoid damaging the original markings and contours. Rotary cotton buffs and polishing compound have ruined a lot of guns.
If you tell us what the exact gun is, perhaps someone can help with more info.