[larsdangly]

I'm not so sure that the standard rules are all that unrealistic. The furthest gap you can cross when moving from cover to cover is 40', or about 7 sprinter's strides. Given that you really need to carefully 'lead' a quickly moving target if you are going to hit them with an arrow or thrown weapon, I suspect you'd find it very challenging to do unless it were a total 'set up job', like a clay pigeon where you know exactly where, when and the path and velocity of the target.

If I were going to make a house rule for something like this I'd: 1) only do it under a lot of pressure, as I would hate to set a precedence for something really outside the core structure of the rules and potentially manipulatable for advantages in other situations, and 2) have it work sort of like an area fire thing where you can loose a missile into a small gap between cover, but with just a gambler's chance of any one missile hitting something.