I didn't teach bang, you're dead the right way. I got him in a down and then helped him roll over on his side with my hand. It worked, but I've since heard that you should try to get him to do the tricks without touching them and forcing them into position.
To do it without using your hands, get them to go into a down, then hold a treat a little toward their back until they roll over on their opposite side trying to get it. Once on their side, treat and start giving the command you want to use, bang or whatever.
I Googled, and this video explains it better.
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Mike ~ Doting Dad to Jillie, Harper, Molly, Cooper, Eddie (RIP), Lucy (RIP), Rusty (RIP) and Jack (RIP).
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