Airsoft guns are clearly NOT replicas under the firearms act.
Replicas do not fire projectiles and are legally intended to simulate real guns for theatrical purpose.
Airsoft Guns DO fire a projectile and are intended for gaming purposes.
It all comes down to intent.
If you purchase an airsoft gun to use as a theatrical prop then it becomes a replica, if you use it to rob a bank then it becomes a weapon.
But if your intention is to play airsoft or simply target shoot in a safe,sane and responsible manner, and you never claim your airsoft gun to be a replica, then forever will it remain not a replica.
Some cop or customs agent may try to bully you into thinking they're replicas, but if you tell them that they're not they really can't do anything.