Landships II

Looks awfully similar to the Boys .55 anti-tank rifle round of WW2 vintage. Rimless belted case in that sort of size - can't be too many other candidates. See http://www.somatech-ab.com/BOYS_55_calibre_anti_tank_reloads.htm. Original Boys rounds are notoriously difficult to obtain.

For more information, including pictures of the rounds, charger clips, etc. see

http://www.rifleman.org.uk/Enfield_Boys_Anti-Tank_Rifle.htm. With a 60 g projectile at 990 m/s for 29.4 kJ it has/had about 10 times the muzzle energy of the .303. It is right up in the same class as the extremely potent WW2 Soviet 14.5x114 mm ATR round (59.6 g 1000 m/s for 29.8 kJ, initial specification). But with a sexier belted case.