The .451 is an elongated .45 ACP; in terms of case construction, it might be closer to a shortened .308. The case head/web area is stronger than .45 ACP, but I don't know if the changes are internal or external. I think guns chambered in .451 were at one time shipped with a reamer, so trimmed .308 cases could be used as a source of brass. The idea was to use a lengthened case to avoid chambering in a .45 ACP, but to keep OAL the same, so it would operate in standard-length actions; along the lines of the relationship of the 9x19 and 9x21. By loading 185gr flat points to the same OAL as hardball, a lot of case volume was created, and this allowed very high velocities without magnum pressures. The .460 Rowland, which was getting written-up in the gun rags about a year ago, seems to be a reinvention of the .451.