Any blowback SMG type of firearm could be turned out in a simple workshop.
rifling isn't really important if its just a spray and pray sort of firearm. I have seen several primitive ones that worked just fine. Of course both 9x18mm and 7.62x25 would be fairly common in Krakow. Probably more of the later. Also 9x19mm from the ww2 era.
Also consider some primitive shotguns, they are quite easy to produce as well and fairly low pressure so they arent soo likely to explode. Steam pipe makes good barrels for shotguns.
Small concealable zip guns for assasination purposes would be fairly easy to make and prevalent in small calibers like .22LR, .25ACP,.32ACP
There would probably be quite a bit of Junk laying around from ww2 like polish and german Mauser rifles in 7.92/8mm Also Polish M44 in 7.62x54R.
It wouldn't suprise me if you turned up 9mm WZ35 RADOM Pistols german lugers, MP-44's even. Here is a link to polish ww2 stuff.
http://en.wikipedia.org/wiki/Categor...fantry_weapons
I think you should consider using an old Grantnik 36 light mortar. That would be an interesting one to use for Anipersonel stuff or armored cars and the likes.
If you want any more ideas or got other questions let me know I am a gunsmith.
Brother in Arms