most modern (after 1900) bullet jackets are copper alloy. the older the bullet, the more likely it is to have a higher copper ratio in the alloy. and as you mention, while there are several salts that can cause the reaction in copper to produce verdigris, almost none of is hydroscopic since they've already been bonded partially to the copper and that's often nothing left to endanger steel alloy. iron maybe, but not steel.verdigris is on the bullet jacket
i've manufactured and sold them - they're an "Any Other Weapon" in the US, and are neat novelty gifts. some jurisdictions have a blanket ban on firearms not readily identifiable as a firearm, but most places have no such ban.I wonder how many James Bond-style pen guns are out there.
they are largely useless in any sort of "James Bond" situation as at the ranges they would be useful at, you are better served with a needle/stiletto type automatic knife for stabbing, or using a pre-charged air capsule to introduce an embolism somewhere vital such as the neck, armpit, inner thigh, hollow of the shoulders, kidneys, lungs, ocular cavity, or spine (between vertebra). from experience, your target is likely not to be wearing armor in all of these locations at the same time in the situations where an agent can be within a useful range.
all that aside, and ignoring fancy ricin pellet umbrellas, it's far better for plausible deniability to arrange for street crime, a bombing, or an unfortunate accident. frequent travelers roll the dice often and mechanical failure of a critical system, especially in countries where PMCS can be costly (training, spare parts, facilities) or impossible (embargoes on the same) is a very real risk.