boy are you confused. 14 or 15, in fact all bowling balls, are the same size.
Throw the weight that you are more comfortable with. with today's balls hitting as hard as they do, a 14 will carry as well as a 15.
the carry is is determined by many factors. I know some know it all will quote you a physics formula involving mass. it is invalid because it doesn't take into account the rotation forces and energy transference.
the single biggest thing to determine carry is, entry angle. ball rotation and axis tilt through the pin deck is next. your ball only hits 4 pins to strike, that is it. so use what ever weight that works for you.
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It was a fine time for me - I was learning to fence, fight, anything anyone would teach me