Get a Dana Wet Rib or Dry Rib (if still available) & attatch it to your pack's lower shoulder straps. Put the handgun in the pouch, where it's readily available. Apply lead as necessary.

The best backpacking handgun (IMHO) Is a Tarus titanium .44 magnum revolver. Light, powerful and absolutely
reliable.

Personally I would only carry a pistol in grizzly country, where it TRULY can be a lifesaver.