I can't get that Napa link to work so I have no idea what Rock Guard is.
A guy I know turned me on to "Bondex Skid-Tex" which is a spray-on texture designed for, among other things, step landings. He painted up some gun parts and ran them through some matches and they held up very well, plus you can vary the coarseness of the texture by applying more or less of it. That stuff seems like its exactly what I am looking for so I'll probably go and buy a can and give it a shot. It seems like the texture is very coarse out of the can, like the grip tape you can buy (also to put down on step landing), but I'm hoping the "roughness" will even out with a few coats of Dura and clearcoat, enough to still maintain a textured finish but dulling the sandpaper feel.
My plan is to rough up the stock with some sandpaper, primer it, paint on the texture, primer it again, and then spray the Dura, with a final Dura matte clear coat to seal it up. Hopefully the finish holds up well. I'll be using Krylon primer as someone on the Hide recommended that.
I'll take some pics of the process and the final product and post them up if there's interest.
I've got three 870s to do as well.