For Bows, one idea from Legend of the Five Rings, is that they have a maximum and minimum strength rating. The maximum strength is the upper limit of damage that can be done with it (i.e. you pull the string all the way back). So if the bow was rated for 50 pounds, and the PC can handle a 100-pound bow, they will only do damage equal to the bow's 50-pound rating. The minimum strength is what is needed ot pull the string back in the first place. If you can't pull the string at all, the bow is just a fancy (and delicate) club.
One idea for the Crossbow is that before battle, the GM rolls vs the crossbow's Strength rating. If they roll over the strength rating, no problem (higher quality crossbows allow a bonus to the d100 roll). But if they roll below, the DM then rolls a D6, D8, D10, or whatever would be appropriate. This is the number of shots before the crossbow comes apart explosively. (This would involve a series of fancy rules for crossbow creation if we wanted, where safer heavier crossbows mass much more than smaller and unsafe crossbows).
Crossbows can be used by people weaker than the crossbow's rating, but reloading them takes longer. For example, a person can easily reload a 50-pound crossbow, but they are working on a 100-pound crossbow. Since there are 50 pounds of difference, the character needs 5 extra rounds to reload. (Exact numbers TBD)