It's a lengthened, strengthened .45 ACP. The "new" .460 Rowland is esentially the same idea; lengthen the case for extra capacity, but load to the same max OAL as the .45 ACP so it will work in standard 1911-pattern guns. Cases can be made by trimming .30-'06 or .308 brass and inside reaming. The .451 is limited to fairly light (read short) bullets, as the case volume must not be used up in deep-seating bullets; that's how the pressures are kept reasonable. Max loads would get a 185 going 1200+ from a 3.5" Combat Master.