Often when giving the firearms safety course here in Toronto, I get asked about reloading ammunition, which lead to creating the reloading course, which we gave for the first time just last Jan. 25th. One of the questions that came up was the different types of rims that ammunition that exist.
There are a few different rim types that you can find on modern ammunition. They are:
- Rimmed
- Semi-rimmed
- Rimless
- Rebated
- Belted
It must be remembered that all cases have rims, but it is the size that makes all the difference. Rimmed cartridges have a rim at the base of the case that is significantly larger than the main body of the case. For example, the .38 Special case has a rim diameter of .440 inches and .379 inches diameter for the body of the case.
Semi-rimmed cases have a rim only slightly larger than the main body of the case. The .32 Auto, for example has a rim diameter of .358 inches compared to the body diameter of .338 inches.
Rimless cartridges are of course not rimless, but the diameter of the rim is about the same size as the base of the main body of the case. The .308 Winchester is such an example. Rim diameter: .473 inches; Body diameter at base: .470 inches.
Rebated rims are smaller than the main body of the case. The .50 Beowulf cartridge has a body base diameter of .535 inches, and a rim of just .445 inches.
Lastly we have belted cases. They are a bit odd-ball, as the case has a thickened area just forward of the extractor groove which is used to headspace the cartridge and isn’t actually part of the rim. The rim of the case can be the same width as the belt, while the main case body diameter is slightly less. The .458 Winchester Magnum is such a case: rim diameter is .532 inches, belt diameter is also .532 inches and the case body at the belt is .5128 inches.
You can learn all of this and more with the Reloading course for Rifle and Pistol, with no prior experience in reloading needed.