Re: From semi-auto to auto
Disclaimer: Me no expert. Me no know much about shooty things. Me no Groot, and so on. Anyway, that said...
I believe 6 bullets is standard for a revolver, a normal pistol will have a clip of somewhere around 10-15 bullets depending on the model and the clip fitted.
Technically I guess what you suggest could be done but a skilled operator can fire a semi auto pistol very quickly anyway - hence firing tactics like double taps.
I suspect it would reduce the learning curve for firing a pistol quickly and accurately, but bottom line is you still have to be able to fire it accurately. In terms of effectiveness you would lose the agility of a pistol; meanwhile you don't gain the effectiveness or range of a rifle.
Even if the weapon is effectively untouched I imagine it would still be illegal once strapped to the gun. It may not be a physical modification to the weapon itself but it is a modification of how the weapon functions. Hence you'd still hear the distant thrum of the black helicopters -->