I think the best balance would be to make the shotgun just as powerful in its own right as the other weapons, but also have it consume ammo at a comparable rate and only spawn with one pickup's worth of ammo for it. Then your extra incentive to find a new weapon is because you only have so few shots with your starting loadout (unless you're good with the laser).
Yes i was just thinking about this now actually before i read your post

This is the best idea, reduce the starting ammo to something not very high, like you say so the incentive is to go get other weapons, good call
