All you need is two, 1" pieces of electrical tape. Stick one across the bore to seal out the elements. Then wrap the other piece around the barrel & 1st piece of tape, this will prevent the 1st piece from peeling back, losing its adhesion or otherwise fall off. .NO it WON'T harm a thing & when you pull the trigger it will be GONE before the bullet can reach it. Electrical tape CAN'T hold back the 25,000-60,000psi that your bullet will produce as it travels down the bore, moving the column of air that is in front of it plus all the heat & fire!