Regulating based on detachable magazine isn't a very good idea.
Keltec is already making a 20 round pistol that doesn't have a detachable magazine and is loaded by clips almost as fast.
The lack of detachable magazine makes it more easily concealable as well because it saves on weight and width.
Feature specific bans do nothing except encourage manufacturers to develop loopholes, and that happens faster than laws go into effect these days.
I do want to give Colorado credit for requiring a background check even on private sales, that's something we've needed to fix federally for a very long time.