If that stuff is in the threads on the barrel nut, which it may well be, it could have caused significant damage.
Savage bead blasts after
assembly. Yep, a shortcut...
But, there's no way for the media to get into the threads of the assembled action/barrel once the nut is in place and torqued to spec before blasting.
Now, loosening the barrel nut if one ever wants to change the barrel would obviously mean it all needs to come out or you'll screw up the threads.