Might need to fit bright orange flash hiders/tips to the barrels, afaik its one of their rules for airsoft stuff. So I would presume they would need you to do that to import.
It's fine as long as you declare it with the airline and it's in it's own secure case. You need to carry your RIF defence with you in written form and it really helps if you are booked to an event or site in the USA before you go and can get a written confirmation.
The airline will note it down and my friend had his RIF and the paperworked checked by a Firearms team at the airport before It was allowed to go through but beyond that there were no issues.
As long as you do everything by the book it's really no hassle.
Mostly US info (it's from the TSA blog) but applies here too. Guns should be fine in checked luggage but you MUST check with your airline. Anything that looks like a grenade is an absolute no-no.