Quoted:
What I'm trying to protect is the design of the lower, simply so that no one can patent it & then make it unaffordable.
I am not a patent lawyer (although I did pass the patent bar exam 20 years ago), but my perhaps outdated understanding is that by making the design public, you will essentially prevent anyone else from patenting it, because your design will become 'prior art' in the public domain that would make any later copy "not an invention". One possible way of doing that may be by making an inexpensive provisional patent application, and then letting it expire in a year, however simply publishing it should be sufficient.
But you might want to couple that with a Creative Commons license which will spell out exactly what others may or may not do with your design. Again, I'm not an expert, but I think that the Creative Commons license applies to copyright (of things such as plans or designs), which is a different type of intellectual property than utilitarian features that may be protectable by a patent (there are also design patents that cover the "look" of an object but not the utilitarian features).
I don't know of anything that would interfere with your use of the Creative Commons license with designs for firearms or firearm accessories.