Stage 18, 251.
I hardly ever use the shotgun, only if there's four or more zombies on the screen and they're getting close.
Don't ever fire more than three shots out of the shottie. Once you fire the last shot, he starts reloading, and you can't do a transition to the pistol in the middle of a reload, which takes for-freakin'-ever.
Also, there seems to be a little glitch that will let you get an extra shot in with the pistol. If you click after you hear the third clicking sound in the reloading sequence, sometimes you will score a hit. There won't be any gunshot, and your ammo won't deplete, but you will hit a zombie. It seems to happen maybe 10% of the time.
Oh, and one more thing: I've got way too much time on my hands.