I've heard of a number of 'pellet' looking airsoft guns make it through because as you said, they look like a pellet gun. However of course that hasn't always been the case.
As always its judged on a case to case basis.
When I got my TM VSR I noticed the word 'airsoft' repeatidly printed over the box. I'd bet it would have a better chance of making it through in a blank box.