This is not the 303 British necked up to 375 but instead the cartridge is the 375 NE necked down to 303 British diameter.  Its essentially a second generation wildcat.  The 303 british was lengthened and necked up to make the 375 NE  and then necked back down to 303 so in reality its a long 303 British.  I have ten original cases from the 1900's but since I don't have the 375 NE or any extra long 303 I'm forming 7mm rimmed Brenneke and necking up.   

Factory loads were a 215 grain soft point at 2500 FPS and a 200 grain soft point at 2725 FPS.  Powder was a proprietary powder called Axite.  It was pretty fast for its day and was tested for its armor piercing capabiilty. 

Handbook of Cartridge Conversions show a load of 53 grains of IMR 4320 which, of course, has been discontinued.   

Case capacity is 68.21 grains of water.   

Anyone have any thoughts - perhaps someone with quickload could give an assist.   

Thanks

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The 300 H&H was basically a good stout 30-06 load. It needed the big case for the tropics, because for some reason, the Poms insisted on sticking with the inconsistent, unstable and unpredictable cordite. Even though there were good stable flake powders available.

It must have been fun in the early days of cordite, I guess after a while more firees got killed than firers, so they knew they had a rifle that would handle the pressure.