I am seriously contemplating registering my S&W 15-22 as an SBR. I have forms in for my .22 can, and I love my 15-22. I also have a 15-22p, which would make the initial build extremely easy. My biggest question is about engraving. I notice the serial# on my receiver is on a small metal plate, but the rest of the receiver is polymer. Is it ok just to engrave the Trust into the polymer?