I have an idea about the Storm PSR being in Zombies. It would be found in the box, and for a normal shot it would be 100 damage, managing to not even kill a zombie on round one with one bullet. However, the longer you charge it up it would multiply the current damage by two.
1 bullet - 100 dmg
2 bullets - 200 dmg
3 bullets - 400 dmg
4 bullets - 800 dmg
5 bullets - 1600 dmg
At first, the damage is laughable, but eventually it becomes a juggernaut of damage when fully charged. Coupling this with the ability to penetrate ANY object may seem overpowered, but I thought that since you would only get, say, 80 bullets with it, this would balance it out. So if you charge it fully, 5 bullets gone right there.
PaP'd would double the output, so starting with 200, ending with a whopping 3200 dmg output when fully charged. You would get 30/160 ammo and a headshot multiplier of five (meaning you could kill a zombie on round 9 with an uncharged bullet and to the mid 30s wth a fully charged bullet with just a single bullet to the head). As an added bonus, the crosshairs would be at a size similar to the ray guns, for an accurate noscope (so as to not risking death by zooming in)