Not sure if you want for AI or actual Player playing the game? One is a little more complex than the other.
For Player if you use waypoints make sure in the text area use something to let him know if he has to pick-up the weapon.
Use a group trigger, change to leader and in condition feild use this:
{_x hasweapon "revolver"} count units group leadername > 0
or what everweapon you are using.
For AI pretty much the same but the waypoint and triggers need to be close with the ammo crate and that may be tough to line up. Make sure you name the AI Loon that is getting the weapon. May want to use a script with AI.
Hopeflly this will get ya started in right direction read Waypoint Tute and Trigger Tute they should have good info too.