The usual accuacry degradation due to shorter barrel length is experienced on iron-sight rifles where shortening the barrel reduces the sight radius. Even then, the rifles aren't mechanically less accurate, the sight system is just less precise in practical use (think full-length M16 vs M4). Naturally, muzzle velocity suffers as well, which means that the bullet will stay supersonic for a shorter distance. It could have stability issues when it goes subsonic. It will go subsonic eventually from either a long- or short- barreled rifle, of course, but the transition occurs sooner if your rifle barrel is short.