How about instead of a barrel tag, a tap/pat on the shoulder/back with your hand?
I can just see barrel tagging going very wrong. At least your hand is softer and you don't have to worry so much about whacking them with your barrel lol. Overall I think barrel tag is a bad idea, but again... it does make sense if you do run out of paint.
(I just keep seeing an image of someone holding their marker like a baseball bat haha).