Regarding this British experimental cartridge, sometimes refered to as the 280/30, in 1950 the Springfield Research & Development Division under a Colonel Studler was tasked to trial several cases of this ammo, courtesy of the UK government and fired in a locally modified M1 Garand rifle.
The function of the rifle was found satisfactory with the gas port opened to .150 (seems like alot but whatever) and the Garand functioned successfully in the regular manner.
There was some problem with the British ammo though:
"During the firing procedure, hot flinders of bullet guilding and burning powder were exhausted back through the gas cylinder against the shooter's hand."
It's lucky for the Brits we made alot of service ammo for them during the Korean War. Looks like they could not even make this ammo to a basic standard and probably used poor materials.