Because that's what the bore measured out to.
bore
Bore is the inside diameter of the barrel. depending on the nation, it may be measured from the bottom on one groove to the bottom of the opposite groove, or from the top of one land to the top of the opposite land (raised part of the rifling). Measurements can be to one thousandth of an inch. Thus a rifle with a .308 bore is .308 thousandths of an inch wide inside the barrel.
A musket is smooth bored, like a shotgun's bore. A rifle has rifling inside the bore (grooves).
The bore is rifled so as to put a spin on the bullet. This spinning gives a greater accuracy to the bullet, especially over that of a smooth bore.
Almost any Combination BB & Pellet rifle is smooth bore. BB rifles usually are Smoothe bore. The Crosman model 760 is such a rifle and can be purchased at most sportingood stores. Look at the packaging and it will say Smooth bore. Smooth bore barrels are less accurate than rifled bore barrels. Cabelas, Basspro, Big5 and Sports Authority usually carry them.
No
Caliber
Rifled, unless you have one marked "shot only"
The bore diameter of a 250-3000 savage is .257
For a rifle it would be the 2 Bore.
Caliber