A short barreled rifle is just a plain rifle, semiauto or fullauto, whose barrel length is less than 18 inches. It is not inherently deadlier than its longer brothers and in fact launches projectiles at a lower velocity precisely because of its shorter barrel. However it is still automatically classified as a Class III item by the BATF, probably because it is easier to conceal than a regular length rifle, that is if you wear trenchcoats all the time.