If you look at a weapon as simply a tool and cost plays a big role in deciding what to use, you are going to use the cheapest most easily available cartridge you can find that will do the job. Sure, the .303 is too light for polar bears - but there was a time once when polar bears were killed by wrapping a sharpened bone bent into a small ball, covered with frozen fat. The bear hopefully ate it fast, the bone would spring straight in the stomach, and then the bear was followed for several days to see if it would eventually expire from internal bleeding. Seems a .303 would be an attractive alternative. There are a lot of marginal weapons - flintlocks, bows - that can be used if one is backed up by a professional. So the circumstances, experience and the resources available play a strong role in deciding what is an adequate choice.