Good underbarrel guns?

My RDB's firerate is actual dogwater and I use a second gun to double my pew pew power because full-auto is banned at my field, however my current secondary (Sharps Bros ar-15 something) is a little heavy. Anyone know of any super lightweight guns to use as an underbarrel? (Self-loading preferred)

Posted this in airsoft circlejerk before but didn’t get too many recommendations there