But it is very loud and a reasonable collection of shells tend to be a bit pricey, bulky, and heavy.
An NEF single shot shotgun weighs only 5 pounds and costs about
$120 at Walmart. Undo one screw and it can be broken down to fit
in a pack.
In CA a long gun cannot be considered a concealable
weapon, so you could carry it unloaded in your pack anywhere except sensitive places and school zones. A handgun has a great
many more restrictions.
IANAL