Bore mop works great. Not sure if you need or want it to be 12 gauge. That sounds too large to me. 50 cal would be a better fit. Soak it with solvent and let it sit in the chamber for a bit. Rotate, exact, dry patch on a nylon brush, inspect.
Would you use a 50cal brush as well?
Im just looking to give it a quick clean before using it , after having it bedded and such just more of piece of mind than anything.