What's to stop anyone from buying the tooling, blueprinting Glock's molds, and churning out "Clock 17" handguns? Or Derettas, or whatever? (Ok, you'd have to use a name that was very different, since trademark protection applies to brand names -- but that doesn't do anything for the engineering.)
I know Ruger stopped AMT from making "AMT Lightning" 10/22 copies, but the "AMT Lightning" copies of the Ruger Mk.II were still on the market even after that. I remember when the .50BMG "uppers" first came out, two or three places started making near-copies within a few months -- some guy from Montana had a table at the WAC show for a while.
Is it useless to design your own rifle and try to market it, or is there design protection short of coming up with something patentable?