#1 It's not blasting media, it's tumbling residue. Savage doesn't blast anything, they use large vibratory tumblers
#2 The threads can't be damaged during assembly at the factory by this residue because the parts are assembled BEFORE they are tumbled.

The large vibratory tumblers mentioned above have a rotisserie-like carousel in them that holds 42 assembled barreled actions. The media used is cone-shaped polishing stones.