Why'd I say your first instinct is probably wrong? Because your first instinct was most likely to be rifle grenade or bazooka round. Bazooka round woulda been sorta close - and those who submitted PIAT were actually on the right track - of course if you'd said M16 Priest Mortar round, you would also have been on the right track.

The right track being... spigot. In this case, the Blacker Bombard, intended to stop invading Germans...

I told you prior to 1945 to save you searching a lot of rifle grenades or rockets.

The colors are... British, though there's lots of flexibility, even in brit markings.

The Red Herring clue was the UAV. Made by BOMBARDier of Canada (hence the Canadian reader names... which were also a clue to help you find that UAV...). The second clue - was of a Blacker Bombard emplacement.

There were two types of round, a smaller, 14 pound, longer-ranged (about 450-50 meters) anti-personnel round, and the larger, 20 pound, much shorter ranged (about 120 meters) anti-tank round. They used black powder as a propellant. It would have taken much courage to fight tanks with that sucker from fixed positions like that.