Shipping gun to U.S.

Okay, I've searched high and low and haven't found much info. For those of you that have shipped rifles to the U.S. for modification or custom work, what paperwork was required and who did you use to ship the rifle?

Thanks!

Okay, just got a bit of help...apparently I need an E15 Form from Border Services...now I need to find someone that will actually ship a firearm.

[twofifty]

...is like shipping coal to Newcastle.