http://www.convert-a-pell.com/" onclick="window.open(this.href);return false; advertises their product as:
I think the .22 Centerfire approach ( http://www.convert-a-pell.com/prod03.htm" onclick="window.open(this.href);return false; ) looks promising.Just insert, load and fire!
Convert your favorite handgun to shoot inexpensive pellet ammunition! No special tools, no disassembly, no reloading expertise required. Will not harm bore, action, or component parts of any handgun.
Another variation on the same theme is the Hammond Game Getter ( http://www3.telus.net/gamegetter/" onclick="window.open(this.href);return false; ) although I don't think the dimensions of the 5.7x28 will allow for an offset rimfire blank.The Convert-a-Pell .22 Centerfire kit enables you to shoot .22 caliber pellets through your .22 Centerfire rifle or pistol, utilizing a patented “O”-ringed primer pocket that assures easy primer insertion and ejection. Calibers available include .221 Fireball, .222, .223, .225 WIN, .22-250, and .220 Swift, all machined to accept a .209 shotshell primer, and .22 Hornet, machined to accept a standard large rifle primer. Each Convert-a-Pell .22 Centerfire kit contains four cartridges. Trajectory of the Convert-a-Pell .22 Centerfire kit will average 2" low with exceptional groups at 20-60 feet and beyond.
One of the issues with standard pellets is that a 22 caliber airgun barrel is 0.218" groove diameter and 0.216" - 0.2165" bore diameter, so standard pellets are "a bit loose."
Another issue is the shape of most pellets. Even if the brass is neck-sized appropriately, the "classic" pellet shape doesn't provide for very stable seating. The Skenko ( http://www.skenco.com/" onclick="window.open(this.href);return false; ) lead-free pellets (and others of similar design) look like they'd work better.
Random ramblings...