My mother was a gunsmith… waits for confused looks to pass … Most (nearly all) firearms are not cross compatible.
Unless they were manufactured with the specific intent of having swapable parts, the best one can hope for is them using the same calibre and cartridge size: 9mm parabellum for example, .45 ACP, or the NATO standard .223.
This is handled nicely (if somewhat simply) in Unturned 3.x as until you fill mags most ammunition is generic within a range of possible calibres. Civilian ammo magically transforms to match the magazine, and firearm for which you load. in reality this includes .22 (short or LR) up to .45 (ACP) calibre.
Proposing an additional layer of part swapping system for handguns and rifles beyond the magical calibre changing attachments (silencers, muzzle brakes, retooled barrels) which already exist as aftermarket add-ons in 3.x is unnecessary complication in my opinion.
As to repairing… I don’t want to play at fieldstripping weapons, I got enough experience doing so during my 12 years in the Canadian Forces.
Cleaning/maintenance of weapons and tools should be fairly simple, like the wearing down of durability and repair system we already have. It’d be nice if there was intermediate repair options that restored a percentage of durability. For example: Gun cleaning kit + gun oil + less than 100% firearm = +25% durability [max 100%] or something. The all or nothing approach doesn’t scale material requirements to amount of repair required currently. The same argument is applicable to crafting food or anything with less than 100% condition materials (1% chesese + 2 x 1% bread = 100% grilled cheese sandwich) - and is probably better addressed in a re-do of the whole crafting recipe system.
Just my 1/100th of a toonie.