I have not seen that one but it looks like the Mecar (of Belgium) subcaliber cartridges for the Blindicide bazooka.
Those had percussion primers and the projectile shape varied as there were different types over the years. Maybe your variant here was adapted to electric priming to be used with a certain weapon.
As per the markings (the “T”) it looks like from a country with British influence.
I have nothing on hand to compare but maybe somebody here can check on the case length of the original Belgian ones.
The measurements you are giving are somewhat confusing as round and cartridge is the same.
Also the rim diameter with 16mm is odd as it is the largest diameter on this round which’s projectile is 20mm in diameter already.
What is the case length if it is not the 27mm you gave as “cartridge length”?
Can you clarify please?