I called a border agent and it really varies by border as airsoft is still not known by every officer. this is what i got
If it shoots under 366 fps with a .20g bb it is classified as a replica firearm and is illegal
if it shoots over 366 fps with a .20g bb and under 500 fps with a .20g bb it falls under a uncontrolled firearm and is legit
if it shoots over 500 fps with a .20g bb it is classified as a firearm and is illegal.
Paper work supporting this is ideal showing fps limits. again this is at your own risk and even with all the paper work you can possible have they can still stop you if they choose.