[Suicide Junkie]

I got 233.

The trick to the shotgun is to use it for emergencies, and then just before the round ends, blast it all away and start the reload.
The pistol auto-reloads for the next round, but the shotgun has to be emptied first.

Emptying the pistol into the air so you can reload is also useful when there are no zombies on the screen yet.

Also, if you just shoot in their general direction, you'll only hit sometimes.
If you click right on them, you'll hit for sure.