I think it takes damn good aim to take out this sort of zombie while on a bicycle! Probably most effective to hit the engine block with a bullet that can break it and jam the engine - this a big caliber, penetrating round, with high velocity and which stays on its line while penetrating lighter steel panels. Plus, how will the bike respond to recoil when moving slow enough to concentrate on shooting? And the Python has only 6, and the invading hordes are much more numerous? The mass of a brace of hand weapons plus ammo is a lot more than a full set of locks!

Storage? All weapons and reloads must be ready to hand, presumably in front bag, enough to take on a three-lane rush hour charge from the baddies. This dictates that for urban road self-defense you need a porteur with low trail but not too much planing, very wide gearset - still necessary to sprint to those islands of cover, where you can set up an effective counterattack in your temporary redoubt.

To say nothing about requisite shooter skill ...

Wow! Two-wheeled asymmetrical warfare!

This is quite a challenging new bicycle design problem!