This is mainly for folks who might not know about rifle calibers irl, to hopefully help you understand the game and what people talk about on here.
.30 caliber covers a wide variety of cartridges that all use a common bullet diameter. In this case the diameter of the bullet is .308.
.300 Win Mag (Canning Magnum in game), .30-06, .308, .30-30, .303, 7.62x54r, and 7.62x35 all use .30 caliber bullets.
Each cartridge has a VERY DIFFERENT power level and general use. .300 Magnum for example is the most generally recommended caliber for animals the size of mule deer, black bear, elk, and moose.
.30-06 is the most recommended caliber for general hunting. That'd be anything from small feral pigs to Alaskan moose. .308 is also a generally recognized hunting cartridge and works in shorter, lighter rifles without excessive recoil
.30-30 is the oldest .30 caliber cartridge still in use in America and it's another amazing general purpose hunting rifle for the same kinds of animals as the .30-06. The difference is the powder capacity of the smaller .30-30 case. It doesn't reach the same distance as a .30-06, so it's typically used for close range hunting within 200 yards generally.
.303 is essentially the 30-06 of England/Britain and their former colonies
7.62x54r is Russia's general purpose rifle as well.
7.62x35 is the weakest .30 caliber cartridge in the game. It's called the .300 blackout irl and is generally used for coyotes and feral hogs.
Compared to their use IRL none of these rifles are classed correctly for in the game use. They are heavily restricted by gameplay rules and poor performance in several cases.