12 gauge shells.
.22 long rifle
It is difficult to find even 10 gauge shotgun ammo today. I have my doubts if 8 gauge shotgun ammo is even made any more, but corrections welcomed.
Standard 16 gauge shotgun shells.
no they are two different sized cartridge's
DO NOT FIRE WITH MODERN AMMO!!!!!!!!!!!!!!! MUST be checked out by a gunsmith.
$4 or so.
Do an internet search and you will find out.
Supply and demand
Do a web search on "12 gauge ammunition types" Over a dozen.
There is a ammo clip for a J C Higgins shotgun model 58324 16 gauge. You can buy it as a replacement part.
Remington makes a variety of rifles and shotguns using various sizes of ammunition.