practice, practice, practice.

That's the jist of what most of these people are saying. If you aren't familiar with your rifle, 100 yrds might be to far.

it also depends on the area that you're in. Out west in say eastern montana or the wyoming sage flats 300 might not be a "long shot".
back east in the hills of appalacia or in midwestern heavy timber, you might not ever take a shot longer than 150 yds.

shoot out to what you think will be the farthest you will ever possibly shoot, and then shoot much farther.

I'm not talking about a box of shells and an afternoon at the range. Lots of rounds. Lots of different days and weather types.

calibers are similar to the draw weight on a bow. The bigger you go 60lb -> 80 lb the more power. The larger caliber the more power. Just a rule of thumb. There are so many different sizes within a given caliber (bullet diameter) so don't assume that a 30-30 has the "power" that a 30-378 weatherby has.