Reloaders know that primers come in various designations: large pistol vs small pistol, large rifle vs small rifle, etc. So what's the difference? Can you interchange some of these different primers? The answer from Brownells Gun Techs Caleb Savant and Steve Ostrem is, "Sometimes." Some interchangeability is possible, but you really, really have to be careful. Just because one type of primer will physically fit in the pocket of a case doesn't mean you should use it. Some handgun cartridges actually do use Small Rifle primers (e.g., .454 Casull) and other highpressure cartridges CAN use Small Rifle primers (e.g., .38 Super). Since the rifle primer is hotter, you'll need to reduce your load and work it back up to make sure it doesn't exceed maximum pressure. Rifle primers have thicker cups, so they can handle the pressure of pistol cartridges, but pistol primer cups are not built to handle the higher pressures of rifle cartridges. A Large Rifle primer is usually too tall to fit in the primer pocket of a pistol case. It'll sit proud of the case head and potentially tie up the rotation of your revolver cylinder or cause a slam fire in your semiauto pistol. Best advice is to use the primer specified in your loading manual. Otherwise, do a LOT of research with credible reloading sources before you substitute primers.