Bullseye is/was a 12ga shotgun powder, not all that disimilar to Red Dot in burn/pressure profile. However, it's unsuitable for laoding the 20ga, as are a half dozen other shotgun powders. Start with a common powder that's too fast for the 20ga to begin with and it's physically possible to double charge such a powder.

I think it's a likelier, and scarier, scenario with a 20 than a 12.