I voted 9mm in semi. It's what I have the most ammo for on hand, and have clips loaded and ready to go.
If ammo wasn't an issue then I would grap the 6" GP100, and all the .357 mags and .38's I had on hand.
If ammo wasn't an issue then I would grap the 6" GP100, and all the .357 mags and .38's I had on hand.