Ooh, ooh. I would propose
a third variety to the earlier pair of examples of custom-crafted weapons I presented earlier ('five guns welded together, with simultaneous triggers' and 'The like five different guns taped together by Ripley at the end of
Aliens').
Perhaps have a universal sort of 'debuff' applied to all weapons made without, say, the big, powered machinery of a workshop or something? Like say, if you disassemble a gun, and try to reassemble it, it ends up not fitting as tightly and neatly together, so you have a 'rebuilt' M16 or whatever, which would have an increased jam chance (does Cataclysm even DO jamming and failures like that? It's been awhile :x), decreased accuracy, etc. You could process a custom gun at one such work places - steps either abstracted away or tied to what parts make up your gun - to (possibly gradually) remove the debuff, as you tighten the fit and whatnot.
Then, you could add workshops around as a new building type, filled with big machinery - lathes, table saws, and all that stuff. Have it be like that Red Jacket place on
Sons of Guns. Except, you know - less gun-centric. Maybe have the same equipment show up in garages and factories - which don't exist presently, but, yeah.
I'm sorta envisioning Metro 2033 and 2034 sorta stuff - it would be crude and less efficient and whatnot, and simply in as an alternative when you can't get a proper weapon, like how the pipe rifle is - or just for stylepoints.
Could be useful for future things, maybe - have Bandit factions toting intimidating-but-stupid things like hexashotguns, or a sorta Metro/Stalker mode like Classic Zombies mode - make proper weapons rarer, arm survivors with cobbled-together weaponry, or something.