Good luck finding anything semi-auto for 500 bucks. Double that for finding the ammunition regarding said weapon.
Hard to get something more powerful in a "readily available" semi-auto hand gun than a 45acp, which you already have. I guess you could look at something like a desert eagle in a 44 mag. If you can find one, it would probably be 3-4 times your budget. What are you looking to take down that a 45acp can't handle in one shot? Grizzly bears?