Rim (firearms)
A rim is an external flange that is machined, cast, molded, stamped, or pressed around the bottom of a firearms cartridge.
However, box magazine firearms firing rimmed cases have seen extensive use; the famous Lee–Enfield rifle used by the UK, and the Russian Mosin-Nagant rifle which uses an interrupter to help prevent the snags, were both used from before World War I until after World War II.
Rimmed rifle cartridge examples include the .22 Hornet, .30-30 Winchester, 7.62×54mmR, .303 British, 8×50mmR Lebel, and .45-70 Government.
[2] As early as 1867 Joseph Whitworth patented a round which "has a solid metal back without any projecting flange, as heretofore, the ring groove already mentioned being a substitute for it".
[3] Even though some rifles were designed for rimless cartridges by Bethel Burton, Eduard Rubin and others, the first adoption did not come until 1888 with its Patrone 88.
[4] Since a straight-walled rimless cartridge is designed to headspace off of the case mouth, this prevents the ammunition loader or manufacturer from using a heavy crimp, which is a ring pinched or "crimped" into the cartridge case, designed to lock the bullet securely in place until fired.
This can be a problem for magnum revolvers or rifles which simultaneously chamber more than one round of ammunition, as the recoil from the firing successive rounds can loosen the bullets in the remaining cartridges, and cause their bullet seating depth to change, which can have a serious effect on accuracy, or could, in the case of a revolver, cause a bullet to protrude sufficiently from the front of the cylinder to obstruct the revolving of the cylinder thus jamming the gun from firing additional rounds.
Rimless rifle examples include the .223 Remington, 6.5×52mm Carcano, 6.5×54mm Mannlicher–Schönauer, 6.5×55mm Swedish, .308 Winchester, .30-06 Springfield, 7.65×53mm and 7.92×57mm Mauser.
An example of a rebated-rim cartridge is the .50 Action Express, commonly chambered in the Desert Eagle pistol.
By using the same rim dimensions, a Desert Eagle could be converted from the .44 Magnum to the .50 Action Express by merely changing the barrel and magazine.
These "advanced primer ignition" (API) blowback weapons feature straight-sided chambers which are longer than necessary to contain the case.
The addition of the belt allowed the cartridge to properly headspace, despite the relative lack of a definite shoulder.
The reason for the lack of a definitive shoulder was that these old British cartridge cases were intended for firing cordite charges instead of modern smokeless powder.
[11] Examples of belted handgun cartridges include the .40 BSA Auto Pistol and .40 G&A Magnum.
More recently, new "magnum" cartridges introduced in the United States have been rimless or used rebated rims based on the .404 Jeffery that fit the same .512" bolt face used for the belted cases.
