You'd want to check the laws to be sure, but there is some precedent for shipping BP weapons into the US. I know other BP gun manufacturers throughout the world export to the US (like the Indian guns supplied by Loyalist Arms (also a Canadian company, or the ones offered by Middlesex Village Trading Company - both of which are from the same supplier). Obviously, these are handguns, rifles, and muskets, so the same rules may not apply, but they are imported without vent holes drilled, and then are supplied to the final customers in retail after they have been drilled. A similar arrangement may need to be worked out, but Loyalist ships guns to the US from Canada that are fully functional.