Electrical tape works well, as long as it is:
A) quality tape (I prefer 3M).
B) not too cold out (in temps below freezing, the tape may not adhere well).
C) kept dry. If you get any water under the tape, it's likely to slowly unravel and come off.
For that reason, I prefer:
Water balloons for typical barrels.
Party balloons for large barrels and/or muzzle brakes.
They're a little more fragile, but you can keep half a dozen in your pocket without taking up any room. I think I've only ever had two balloons get cut in the field - one when the muzzle slid down a rough tree trunk during a bladder draining session, the other when a branch punctured the balloon between baffles on a large .45 cal muzzle brake.
(I turn my balloons inside-out when I put them on, to keep the abrasive powder from getting inside the barrel, chamber, and action. -It's generally talc or powdered granite.)