It does not set up rust anywhere, because the rest of the gun is masked off with wax, If the bore was not wax masked then it would be plated as well wouldn't it, rust would be the least of the worries !! Once the leads are attached the whole gun is a conductor.

So whatever plater you spoke to is an idiot.

This was not a theoretical response. We do exactly this, several times a year for clients, forum members amongst them.

It is by far the least damaging and most cosmetically invisible tightening job.

Regards