The .22 short, long, and long rifle all have a heeled bullet, where the diameter of the bullet (and barrel) is the same as the diameter of the case (and chamber). Therefore, you can fit a .22 LR into a .22 short chamber, as long as the rifling doesn't make it too hard to push the bullet into the chamber, or actually hit the case. The gun may not be able to hold up to continuous use of the higher pressure .22 LR over the .22 short.