Typically, gas which follows the projectile down the barrel will be held back by the projectile. A small hole at the top of the barrel allows some of the gas a path of escape. This gas presses against either the piston (in a gas piston rifle) or the bolt (in a direct impingement rifle), pushing it back and working the action.
Barrett makes a range of .50 rifles, some of which are self loading (semiautomatic), and some of which are bolt action. The most famous of these rifles is the M92/M107, which is semiautomatic.
Millions have been made.
Yes, but there ARE restrictions on WHICH rifles, as well as which features are permissible.
Some are, some are not. California has very stringent limitations of firearms.
No. Semiautomatic .22 rimfire rifles may be used for small game, but no semi rifles for deer. See link to Vermont hunting regs, and scroll down.
Repeaters
It can, but, damage will eventually happen. Not recommended.
No
Yes
Contact Savage. They are still in business, making good rifles.
DO NOT SHOOT AMMUNITION IN A WEAPON NOT CHAMBERED FOR IT. If you do, bad things will happen.
Repeaters