The basic idea is to string a piece of cable between two trees, that a target on a pully system can run down so I can shoot at it.

Design challenges:

1. How to trigger the target pully moving.

2. Is there any way to not have it sloped down i.e. a gravity powered pully on which the target hangs

Any other design suggestions appreciated.

I don't know if this fits your application, as it is more geared toward precision rifle training, but I was watching a sniper program on the History Channel once, and an underfunded department came up with a pretty inventive and cheap way to train for hostage scenarios.  Basically, you tie two different colored helium filled balloons to an RC car; one color represents the hostage, another represents the perp.  The balloons are tied off with a length of string that would approximately simulate the height of a grown man, and since they're filled with helium, they float.  Then, one person operates the RC while the shooter must engage the hostage taker balloon before the RC driver gets both balloons behind cover.

Any other ideas?