what does the feat do? it is supposed to give same number of attacks with crossbow as with other weaponss. so how is apr calculated for ranged weapons and how is it different for crossbows? i thought it is same no matter what weapon and depended on bab. but if so the feat doesnt seem to make sense.

AFAIK crossbow's are limited to 1 APR regardless of who wields it (don't ask me why - must be to do with the higher damage and improved critical range over a longbow). Taking rapid reload just removes this cap so you'd get the same numebr of attacks as your BAB's allow (which will therefore become the same as a longbow or shortbow).

APR with long/short bows is identical to when wielding any other single weapon.