For the most part, yes. Federally it is legal (as long as there is an orange colored tip or plug in the muzzle) but state to state the laws are very inconsistent, or should I say, very convoluted. Some states require that orange tip permanently affixed to the muzzle. Some only require the tip during sale and transportation but it can be removed otherwise. Some states allow you to have an airsoft if it isn't "colored" like a gun- black, silver, etc. Some places like DC and San Francisco (of course ) ban them outright.
Last year I ordered a full metal airsoft replica Beretta M93r for a Robocop Auto 9 build. From Taiwan to California and I had no issues as far as customs goes.
Really there's only a handful of states that are strict about it. Most other states you're good to go.
You may be interested to know that you can already get the Auto 9 in metal (with a plastic sleeve) that is easily modifiable to fire 3 round burst with the purchase of a small part sold alongside, just google KSC airsoft auto 9.