Unless you (or your grandfather) shot cast bullets in that gun there should never be any lead fouling in the barrel. Use a copper cleaner and use it with gusto.

I assume that it' s a pre-64 M-70 and I wouldn' t alter it for nothing.....

If you want to adapt it to a little better weather resistance, buy a new stock (McMillan comes to mind) and glass bed the barreled action to that stock for weather rersistance.