A few have posted that reloading components for the 303 Br are hard to find and expensive. This is certainly not true for any of the guys I know who reload. All the brass you want is available at every range in Canada for free, powder and primers cost no more than for any other cartridge, and .311-312" bullets are easy to find and cost the same as most 308", less than some.
As well, 303 bullets are specifically designed for 303 velocities. Consequently, they perform very well on big game, as do their cousins designed for the 30-30 Win.
As far as killing power, included in the many hundreds of thousands of big game animals taken around the world using the 303, the world record bull moose was shot in 2013 by an acquaintance up here using his "old" SMLE. The rack has an official point score of 263 1/8.
The accuracy of most 303s that have bright, sharp, rifling is perhaps better than 99% of hunters who head out every Fall with their rifles that they have sighted in and shot less than a box of shells though during the past year.
Bones6696, for what it's worth, I would buy one or two boxes of ammo and give the rifle a try. You may be surprised at how easy it is to shoot and how accurate it is.
Okay, we're getting ready to head out for moose and caribou. Taking my 9.3X62 and my wife her 308 RSI, but we could be taking one of ant number of different rifles that all of you use successfully every year to replenish your freezers. Good luck to all of you!
Ted