"Average" is a matter of opinion, and probably impossible to define. In regards to "regular" magazines, some 1911 style .45 pistol magazines hold 7 cartridges, some hold 8. Some 9mm pistol magazines hold 7 or 8, some hold as many as 16, many hold 10. Some .40 caliber pistol magazines hold 8 or 10, some hold 15.
The .357 SIG pistol cartridge is the product of Swiss-German firearms manufacturer SIG Sauer, in cooperation with American ammunition manufacturer Federal Cartridge. The cartridge is used by a number of law enforcement agencies and has a good reputation of accuracy.
The most popular shotgun gauge by far is 12-gauge. The larger 10-gauge, once popular for hunting larger birds such as goose and turkey, is in the decline with the advent of longer, "magnum" 12-gauge shells, which offer similar performance.
The .223 Remington (.223 Rem) is a rifle cartridge. It is commercially loaded with 0.224 inch (5.56 mm) diameter jacketed bullets, with weights ranging from 40 to 85 grains (2.6 to 5.8 g), though the most common loading by far is 55 grains (3.6 g).