To add a little bit more to what Bob33 said, many of the short mags and compact mags use a fat case without a 'belt' such as the 404 Jeffery case then shortened and necked to whatever caliber. Not having a belt allows the body of the case to be the same size as a mag case with similar size belt. The only belted cases I know of that actually require the belt are the 300 H&H mag and the 375 H&H mag. These two actually headspace on their belts. The rest have a belt purely for looks because they headspace on their shoulders. This belt takes up space in your magazine and in many, this only allows for 3 in the magazine w/one in the chamber. With the short mags without belts, there'd be no difference in magazine capacity. Most compact mags are shorter cases than their older brothers and as Bob33 said, this equates into a shorter(less steel)action with similar ballistics. It's also a good excuse to buy another rifle huh Bob?
