Interesting 300 H&H cartridge

[Lee Enfield]

Sunray, thanks for finding a picture of my cartridge, the head stamp reads SUPER-X 300 H&H MAG. Keep in mind the case is aluminum and
the bullet is plastic not a normal copper jacket.

Any indication what this cartridge is used for ?

A red base and bullet is the common indicator of a high pressure factory proof load.

http://www.ammo-one.com/Proof-300H-H.html