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British Cartridges
300 H&H Flanged (Rimmed), also called  Holland's Super .30 Flanged
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On the above Pictures:  Picture on Bottom Right:  (Left) Rimless Belted Case, (Right) Flanged or Rimmed Case
This is the flanged or "rimmed" version of the 300 H&H Magnum. The 300 H&H Flanged (Rimmed) & Belted (Rimless)
Magnums were introduced in 1925, by Holland & Holland of England.
Only a few custom rifle makers produced rifles chambered in this caliber in the U.S.A., and most were in the rimless version.
This caliber had limited popularity and reviews were very mixed with some people saying it was  the best 30 caliber made,
why others saying only as good as the 30-06. The rimmed version was used in single and double barreled rifles and the
rimless version was used in bolt-action rifles.