Developed and introduced by the Peters Cartridge Company in 1920, the 45 Auto Rim uses a modified 45 ACP case that allows owners of 1917 revolvers (chambered for the 45 Automatic cartridge) to use those guns while loading and unloading the cylinder conventionally, rather than having to use half- or full-moon clips. This significantly eased use. For target shooting and plinking, it was a better method. For use in battle, loaded full-moon clips were a wonderful adjunct that speeded unloading and reloading to the point that a practiced shooter could fire the revolver just about as fast and often as an equally skilled shooter could run the 1911. Other than having a thick rim, the 45 AR is identical to the 45 ACP case. However, some evidence suggests some revolvers originally chambered for it were made with unusually soft steel. For this reason, most loading data for the 45 AR is limited to a bit less pressure than 45 ACP data. Modern 45 ACP chambered S&W revolvers can certainly handle more pressure and loads in those launching bullets up to 250-grains at about 1000 fps offer significant fight-stopping capability along with superior accuracy potential. The latter point stems from the reality that most mid-range loads in revolvers necessarily use a propellant charge that fills only a small percentage of the available space (propellants that would fill the case to generate the same velocity do not ignite and burn uniformly at such low pressures). As such, the charge can settle anywhere in the case. Depending upon where the charge happens to be located in the case when the primer ignites, ballistics will vary significantly. This is called charge-position-effect and limits accuracy with such loads. The smaller usable capacity in the 45 ACP and 45 AR cases significantly reduces or eliminates this effect and results in superior accuracy with such loads. As with any revolver round, best practice is to apply a roll-crimp after seating the bullet. A roll-crimp eases loading of rounds into the cylinder and can help limit bullet pull under recoil if only modestly. I cannot too-strongly recommend getting a second seating-and-crimping die so you can have one adjusted to only seat the bullet and the second adjusted to only crimp the case mouth. Generally, attempting to do both operations in one step is a recipe for damaged and destroyed cases.