Far as I can remember, Dominion put out 3 diferent kinds of 6.5mm ammo:
6.5x55, which has a base diameter of about .473" and works really well in 6.5 Krags and Svedish Mausers. If you get desperate, you can make it out of .30-'06.... or you can buy it from any of the American manufacturers (who now have discovered this round). For many years, this was the only 6.5x55 available in North America.
6.5x53 Mannlicher-Schoenauer, a sporting cartridge adapted from the Greek 1903 military round, which was a slight modification of the Italian 1891 cartridge although with slightly lessened pressure intensity. This is the world-famous elephant-killer, whereas the more powerful Italian round is well known as a total piece of crap. Something around here doesn't make sense, but that's the way it is. You can make this one frm .303 Savage, given that you have a lathe. For a LONG time, Dominion was the only North American source of this round, which was de-listed in the States in the '30s.
3. The mysterious R cartridge, which was made for this rifle and sold in pink boxes of 20 rounds through Eaton's. Only headstamp was a capital letter "R" at 12 o'clock position. I have a single round of this stuff, will dig it out and measure it tonight, post the exact measurements later.
Ammunition marked "D.C.Co." is pre-War Two.
Ammunition marked "DOMINION" is from shortly after War Two, to the early-middle 1960s.
Ammuniton marked "IMPERIAL" is from about 1964 or '65 through to the end of production by CIL in 1968. The "IMPERIAL" name was continued by IVI and some of their early production in these calibres was leftovers from CIL. The CIL stuff was great ammo, but IVI had no idea whatsoever of quality control and their product deteriorated until it became posiively dangerous, at which point I wrote a newspaper article in Newfoundland which was printed in at least 11 Nfld newspapers and copied in Nova Scotia (without permission but with my thanks), at which point IVI finally took notice, tried to sue me for half a megabuck and the publisher for another 500,000; they pulled their horns in when confronted with several hundred rounds of their own defective brass. IVI pretty much pulled out of the Canadian civil ammo market, alowing the evil Americans to dominate it, despite punitive taxation measures (prompted by IVI whining) designed to keep them out.
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