I looked at the 17 FB, but I would be concerned about the barrel life with the extra speed. If you want an inexpensive (sorta) option for 20 caliber, then consider 20 Practical. It has about the same ballistics as the 204 Ruger, but you just make your own brass by necking down 223 cases, which are in abundance normally on the ground where people go shoot. I have a White Oak Armament barrel which they normally stock (they are a wildcat round, but many barrel makers off them), in an AR upper, and everything else is common to the AR - BCG, magazines, everything same except the barrel. That's a good coyote or P-dog cartridge and cheap to load.