Trijicon Night Sights.
If you have never tried Tritium sights, try not to. If you do you will want them on all your firearms. In low light situations they are the best I have tried so far.
These are not cast, they are made from milled steel and they must be installed by a gunsmith. Don't go cheap on the gunsmith either.
They don't change the profile of the gun and they add so much utility they are worth it.
--Remember, Zombies are most likely to attacked in the dark!