Thing to do is learn your types and your headstamps
D/!\C is the Dominion Arsenal, the Government plant. You have several DC Mark II and Mark VI rounds there, some dated by month as they were for a short time.
ONE of the ones with the points "ground off" looks very like a factory job. Turn it over and look at the base: it could be a Cordite Mark IV or (more likely) Mark V. These are the "Dum-Dum" series as made in Canada. Canada DID make the Dum-Dum Mark III also but specimens are exceedingly rare. I have the headstamp on a case which was converted to Blank. I also have English and Canadian Dum-Dum Mark V rounds.
R/!\L is the Royal Laboratory, located at the huge Woolwich Arsenal site in London. They made some of your Mark II rounds. Note on all of these the C on the base: Cordite.
DC 16 is Dominion Cartridge Company, 1916: Boxer-primed ammunition made on contract for the Government but using the commercial specs for the casings and powder, military specs for the slug.
Your single round with DCCO and .303 BRITISH likely is inter-war stuff. Dominion did make commercial Mark VII spec ammunition for some time. It came in a blue-and-yellow box and was marked as suitable for Lee-Metford, Lee-Enfield, Ross and Winchester rifles as well as for Lewis and Vickers machine-guns. A fairly large contract of this went to Newfoundland shortly before the Second War, being that Newfoundland did not have its own ammunition plant and so imported from the Canadians.... who were foreigners at that time.
You will note that much of the old stuff is dated. Some dates are VERY hard to find.
DI is Defence Industries, a Crown Corporation which operated 5 huge ammunition plants during the Second War. For their .303 plant, they borrowed the core staff from the Dominion Cartridge Company and filled the plant out with unskilled workers who had to learn on the job. This worked out MUCH better than it could have and they turned out what I believe to be the BEST .303 military ammunition ever made, more than 3 BILLION rounds of it. The rims (vital for headspacing) all were right at the specified Max or just 1 thou short of it, the brass was excellent quality and extremely uniform in all respects and the primers were non-corrosive and non-mercuric, whereas the Government plant stayed with that huge British primer which was corrosive and mercuric both. DI ammunition all displayed a Z on the case-head because they did not use Cordite, they used a specially-made powder based on Nobel Neonite. It was quite similar in burning characteristics to what we now call 4895 and also was a single-base powder. Near the end of the War, DI also turned out some ammo for the US: the ONLY NCNM military 30-'06 ammunition of World War Two. This is the NICEST darned brass you EVER reloaded! When DI made Specials, they were NOT stamped on the case-head, but instead the bullet tips were colour-coded as US/German practice.
The GP-B rounds are Gallery Practice, Ball. Several are correct but one has been 'handloaded' likely with a .32-20 bullet. These were regarded as safe on indoor rifle ranges, developing only about the velocity of a .22, although the slug was 115 or 120 grains as opposed to the 40 grains of a .22"LR. Britain experimented with gallery rounds and developed two different ones, but Canada was the only country to standardise one for Service use. There also was a Canadian variant with a lighter RN bullet, but they are scarce.
I note that some f your Mark VI ammunition is dated 1912. AFAIK, this was the final date for manufacture of the Mark VI, the Imperial standard having changed-over to the Mark VII on its adoption in 1910. Canada was using the Ross Mark II at that time, and all of the early Ross sights were designed for the Mark VI round, so that is what the Militia used until the Mark III rifle came into use. Mark VII ammunition also was made in Canada in 1912, so there is one to find.
Mark VI ammunition used the 215-grain RN bullet at 2060 ft/sec nominal MV. MV for the Mark II with the same bullet was 1960 ft/sec. Specs for the Mark VII were a composite 174-grain pointed flatbased bullet at a nominal MV of 2440 ft/sec.
You have a good start on a collection.
Enjoy!
