What are the great Canadian designed cartridges?
That's a question that many are either going to misinterpret or conflate.
First will be those saying ".303 British". Probably the most significant in Canadian history given it's use by Canada in all the wars from the Boer War to the Korean War. But it wasn't designed either in Canada or by a Canadian. The .303 British was a long established round before James Parris Lee, for a few years a Scot turned Canadian before finally settling in the US, gave us and The Empire his Lee Enfield rifle chambered in that very, very British, not Canadian, cartridge.
Others will name Aubrey White and his Imperial Magnums (and I've sat and talked with Aubrey about how he designed those cartridges after retiring from the Force and later bought two of them). Or the Epps variants of the .303 British. But both those families of cartridges are modifications of a pre-existing cartridge; essentially, giving the parent cartridges the P.O Ackley or Rocky Gibbs treatment (however you prefer to think of blowing out cartridges, their shoulders, etc. to increase case capacity of the original).
As best as I can recall, as far as commercial Canadian designed cartridges, there is only one possible winner. Even if that's because to the best of my knowledge, there is only one Canadian cartridge/cartridge family, and that would be the Ross.
After winning contracts for his Ross rifle for the military and RNWMP, somewhere around 1904 or 1905, Ross started looking for a chambering that had legs for both competitive shooting like at Bisley and for hunting. He started out with a semi-rimmed cartridge he called the 28/06, but it didn't deliver anything better than the already existing 7x57, so he abandoned it; I'm pretty sure it never made it to the commercial market.
The ultimate result was what we know today as the .280 Ross; 100% designed in Canada and manufactured in Canada at the Ross Rifle factory. Not a modification on a preexisting cartridge.
It's not a shabby poster child for Canadian design either. Like the Avro Arrow, it was well ahead of the pack in it's day. Right from the word go, where the Ross Rifle was proofed to 28 tons, about 62,000 psi... few if any modern rifles chambering magnum cartridges are proofed to function at the pressures that the Ross Rifle was.
Compare the 280 Ross of 1907 with it's 146 grain bullet at 3,050 fps to the modern 7mm Remington Magnum that manages to eke out only about an extra 100 fps with 140 grain bullets. 280 Ross rifles loaded with 180 grain rounds at 2850 fps went to Bisley in 1908(?); pretty much the same as the 7mm Remington Magnum with 175 grain loads. The Ross pretty much cleaned everybody's clock at Bisley for a while. Which resulted in the Ross promptly being regulated out of competition at Bisley. The USSR running boar team used rechambered Ross Rifles to win the gold in the Rome Olympics.
The 280 Ross cartridge inspired a lot of advancement in cartridge design elsewhere in the world after it was released to the market in the pre-WWI world. Charles Newton designed his own early magnum cartridges and rifles after looking at the 280 Ross cartridge and rifle.
My brother has a pretty pristine 280 Ross sporting rifle that he still regularly uses to terrorize the local deer and antelope, he's still well above average shooting with the factory aperture sights. He will also tell you that the curse of the 280 Ross is getting hunting bullets of the proper size.
But as far as great Canadian designed cartridges go, the 280 Ross is the hands down unquestionable winner. On performance alone, not the fact that it doesn't really have any Canadian competitors.
