I see a good number of 303 rifles at all of the ranges I am a member at, and I saw a lot growing up. It is a very popular cartridge in Canada, due to the huge amount of surplus rifles that were available cheap after the wars. When I was starting out hunting, you could purchase a surplus rifle for $20, and a sporterized version for under $30. But I honestly have never seen any rifles chambered in 303 other than the surplus rifles.
As for the high number of 6.5CM rifles at your range, your city is full of higher wage earners, that can afford the latest rifles/cartridges. I see a fair number at one range that I am a member at, but far fewer at my local range.
Bit of a side bar but cool story about a 303; My dad bought a 303 for $5 off a train, out of a crate of rifles packed in grease that came right from the war. Full stock, bayonet and everything. Unfortunately he had it "sporterized" to use for hunting. (Unfortunate only because it would be a super cool piece now). Shot my first couple deer with that gun myself.
Back on topic, despite the nostalgia, I prefer my current rifles to that cartridge and now own a 7mm wsm, 7-08, and 30/06. Nothing against it, I just "don't feel the need to creed".
JP