3M electric tape. Keep a roll in the pack and/or a few wraps on the barrel as a "spare".

The 3M sticks very well, is tougher than a ballon, saran wrap, or any kind of latex condom thingy. It will not come off in sub zero weather like a lot of other cheaper electric tape does.

Large bores like a 50 cal ML I make an "X" over the muzzle then wrap one wrap around the ends to hold it all in place.

Cheap insurance to KNOW that nothing is in your barrel, and it will NOT affect your point of impact at all..............