Free: Contests & Raffles.
I use electrical tape. Cant remember where I read it, but some hunting magazine actually did some testing to see if it affected accuracy, using some sort of shooting machine. They used 5 different calibers from rimfire up to a large magnum if I recall. Funny thing was, 4 out of the five shot BETTER 3 shot groups when they had a piece of tape over the end of the barrels before each shot. The 5th gun was an almost identical grouping with or without tape. Was kind of a cool article.
I would not recommend an earplug in your barrell!!! Stick with the sticky stuff like electrical tape. It's proven and cheap and no barrell plugging.