Of course the handgun would only be a backup for your real defensive weapon. Something with major boom power...
But there are some little vicious little things in the park that would need attention too. Things that you may not wish to waste your main weapon's large, heavy munitions on. Things that you would want to use a fairly capable round on, but which would not require an elephant gun to kill. Thinks Compy's. They wouldn't require much more than a hit from a .22 lr to disable, but the calibler should be able to replace the larger weapon in an emergency. So a light, high capacity, ultra reliable handgun in a fairly capable center fire caliber would fill the niche here.
At last a use for the 9mm!
Make mine a Glock 17 with a pair of 30 round mags to accompany the 6 regular high cap mags stuck in my belt. Hey, what about a full auto switch too? Wouldn't want to face a raptor in one of those "emergency" situation's otherwise... Ok, maybe a 40 S&W would be... Naw, 10mm. Yep, make it a 10 mm.
