10mm
I have read this forum for a long time and finally today, starting posting(on duty, nothing better to do).
As for a semi auto hand gun a Glock 20 SF would be the best choice hands down.
Can be easily converted to .357SIG, 45ACP, 9MM…
The FBI said it, I’ll say it. 10MM is the best auto round available
It can be loaded to 40, and can be loaded past 357MAG