" The USCG does not consider it a firearm"
In the US, the USCG does not ordinarily make or enforce any rules concerning "firearms". (If they came across a gross violation of federal code, they could, but that's not something that ordinarily
happens.) The only time the USCG may take issue with a definition of "firearm" is if some boarding team asks whether you have any "firearms" on board.
However, under various state and federal laws, any device that launches a projectile of any kind, using any type of stored energy, whether that is compressed gas, springs, or explosives, is a firearm.
That's the general definition used in many places in federal statues and state ones, not my opinion but written laws.
And while many states, and some cities like Chicago, New York
, and DC (all notorious for their own municipal gun laws) may turn a blind eye toward "pyros" in general, they also have laws to enforce. Take a flare gun on the subway and get stopped with it, and you'd better be able to prove that you're coming to or from your boat, or you're still going to be busted for firearms possession. Feel free to debate that with the judge, because you'll still spend the night in jail.
As to shipping
the flare gun into the US? The USPS and every carrier have their own rules about shipping
and pistols, some regulated by US code, others added by the carrier. Stick a firearm of any sort into a random box and "ship it", and you're rolling a set of loaded dice in this country. Offhand, that's probably a $5,000 or $10,000 fine even if they decide it was an innocent mistake. Of course, shipping a can of hair spray by air can be illegal too, if you don't meet HAZMAT regulations
. And again, the carriers all add their own on top of it.
Bottom line, if it looks like a gun and works like a gun, in the US it may still be prosecuted as a gun.