Military models are a bit of an oddity when it comes to model railways I find.
I have a great interest in them and am working on a WWII layout at the same time as my other, civvy, layout. But walk into a model railway shop and you generally get blank looks!
At the moment the best I've managed to get is some Mk.1 coaches which I am in the process of converting into hospital carriages, which isn't too painful, but still can't be done 100% accurately with the base models we have to start with.
Rail guns are another good topic and on this forum I have seen, a few months back, an excellent steel barreled mounted gun..but that was at least part scratch built. Which is a shame as I have absolutely no metal-work skills at all in which to make my own.
Liliput has a good range of Flakwagon and others if you are after some German models, see HERE
Hope that helps a little!