It's built off a cut down .223. Much like a 9x23 only longer, necked to .224". A 40 grain bullet at 2050-2100 fps. runs a 7 lb recoil spring, no real recoil. It's LOUD. It shoots well, the widebody RIA pistol is quite nice for the money. Horrid plastic grips and mainspring housing, 2 stage nasty trigger pull. Great slide to frame fit, well done and even parkerizing job, well machined.
