British Cartridges
    .300 H&H Belted Rimless Magnum , aka 30 Super
Flanged or
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The 300 *H&H Belted (Rimless) & Flanged (Rimmed) Magnums were introduced in 1925 by Holland & Holland of the UK.
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 sayingit was  the best 30 caliber made,
and others saying only as good as the 30-06.
rimmed version was used in single and double barreled rifles and the rimless version was used in bolt-action rifles.
* H&H is short for Holland and Holland, a British Firearms manufacturer since 1835.