"What are you shipping?"
"CNC Mill."
"Great, we'll be happy to ship that for you."
Don't try to confuse the customer service rep by saying things that have the potential to confuse them; they don't need to know what it's meant for.
It's doubtful that UPS, at a high level, cares enough to ban this. Rather, someone heard "something something GUN MACHINE," doesn't know anything about the relevant law, and made a snap decision that was least likely to get them in trouble.