It doesn't matter why its true, its true. To make the point though, the elements which come into play relate to bore capacity, which is a measure of efficient powder burning in a restricted space. Bore capacity is effected by powder type, bore groove diameter, pitch of rifling, sectional density of the bullet, allowable pressure, and the shape of the case. With the exception of switch barrel rifles, in a given rifle we have no control over groove diameter, rifling pitch, allowable pressure, or the shape of the case, but we can make choices as it relates to the bullet's SD and the type of powder we use. With few exceptions, the powder that produces the highest velocity with a given bullet in a long barrel, produces the highest velocity with that same bullet in a short one as well. Sectional density increases with bullet weight, and efficient powder burning generally improves with heavier bullets.
