Common assumption; however, the only criteria for a cartridge being a "magnum" is having the word "magnum" in the manufacturer's cartridge name. It is really only a marketing ploy that was started in 1912 with the introduction of the 375 H&H Magnum and carries on to this day.

It is a common, but loose and unreliable, method of sorting the more powerful cartridges in a caliber grouping from the less powerful cartridges in thatgroupingand many people get their panties in a knot about it.... Rigbys, Lotts, STW's, STA's, Ackley Improved's, Dakotas, Nitro Expresses, BMG's, etc, aspowerful as they may be,areNOT magnums because they do not have the "magic" (and divisive) "magnum word" in their title.Frequently, the first ones toutter the "magnum-itis" word are the first NOT to understand the difference.