Mech-Tech is good only if you don't want to change it back to a pistol, ever. To do so you must have a tax stamp. Only T/C has a current exemption on one of their rifle kits that allows it go back and forth. No telling who might be at the match and witness you change from rifle to pistol, and get you in hot water for it.
Yes, Redneck with a 40. There is a 10mm AR carbine, Olympic Arms makes it, or you can get the kit form.
For a project, Lone Wolf makes an AR lower that accepts Glock magazines. You can get it in 9mm. .357 SIG, or .40 cal. With the abundance of aftermarket and factory magazines, some over 30 rounds, it might be a good thing to look into.