NEVER fire a rifle with oil or grease in the bore, especially grease! Grease can raise pressures dangerously, and oil will quite often make your rifle shoot someplace different from where it will hit from a dry bore!

When I go to the range to zero a new hunting rifle, I take all my cleaning equipment with me, so I canclean and dry the bore after every shot. Then I let the gun cool to about the temperature it was at the beginning of the session. (This procedure is only for rifles that I know shoot into a different group center when fouled and hot than when cool and clean.)

A clean, dry bore is best for zeroing, since it lets you see where the bullet will go from a clean, dry, COLD bore when hunting. This is called the "sniper's zero", and when firing at a target that can return fire or leave the area when you shoot, the first shot is the most important shot!