That round you have, is very interesting, I have never seen one before, but it does not surprise me that spent cases were put to good use.The British purchased from the US, in gold, many .30 caliber weapons to make up for the loses following Dunkirk, these eventually went to the Home Guard, along with early Thompson shipments.
The Home Guard was equipped with the following .30 weapons:
M1917 Browning Machine gun on M1918 tripod
M1915 Colt Vickers Machine Gun
Marlin Machine Gun, this was a WW1 tank gun
M1918 Browning Automatic Rifle
M1917 Rifle Enfield
M1 Garand, these were very early versions.
These all came with spares and a large quantity of amunition
Here are a couple of pics of two, .30 machine guns, they belong to a friend of mine who has many Home Guard items.The red band denotes .30 cal ammunuition