Kind of rare? I'm sorry but a pistol that fires a non-standard cartridge (which itself is expensive), with a price point over $1,000 is going to be REALLY rare. You must live in a really rich neighborhood then.
why would it be useless against unarmored zombie heads? tell that to FN? if AP rounds were so effective against soft targets, why did they waste time developing spoon tip bullets to specifically be more effective against armored targets.
9mm lacking in stopping power? it's that way for balance reasons if you've read Maxx's posts. 9mm is perfectly fine against soft targets, and standard AP 5.7 would be even less effective than 9mm, arguably (inb4 wound vectors)
It's available to civilians, but the primary market is LE and Military. FN just makes almost all of their products available to civilians in semi versions (don't tell me the SCAR is a civilian weapon solely because the SCAR17S and SCAR16S exist, they were originally designed for the military)
M67s being hard to find? my ass. They'd be everywhere and there are probably more M67s produced currently than Five-seveNs will ever be produced.
Would it ruin the game? no. Would it be a completely redundant, out of place, addition to the game? hell yes.
I have something against people suggesting just their favorite guns to be added to games in which they totally do not fit the settings.