Who are the most famous Manufactures?

Federal and Winchester.

how is the gauge calculated??

An n-gauge diameter means that a ball of lead (density 11.352 g/cm3 or 6.562 oz/cu in) with that diameter has a mass equal to 1/n part of the mass of the international avoirdupois pound (453.59237 grams). Therefore an n-gauge shotgun or n-bore rifle has a bore diameter (in centimeters) of approximately