First before I forget a pretty good site for just such info is www.chuckhawks.com .
The .243 Winchester was derived from taking the .308 Winchester, (7.62 NATO), and necking it down to .243 caliber, (or 6MM). It is typically considered one of the better varmint/deer cartridges. Because it is commonly used in both capacities it is loaded with a large variety of bullet weights. It is imperative to match the correct bullet to the intended use!!! That is probably one of the biggest problems with the .243 Winchester. PEOPLE USE THE WRONG BULLET FOR THE INTENDED GAME! Traditionally the best bullet weight for use on deer size game is 100 grain. Perhaps an even better choice available today is to use one of the premium bullets. (For instance: 100 grain Speer Grand Slam, Nosler Partition, Barnes " X" , etc.) These bullets will give you expansion and sufficient penetration to reach the vital organs on animals the size of deer. (In the hands of a very careful hunter it can, and has, been used on heavier game. But think of that use more as an " Expert' s" gun....not a beginner.) For use on varmints, (where you are NOT trying to save the hides), select nearly any of the lighter bullet weights. Most of which are extremely frangible. Pick the one that shoots best in your particular rifle. If you decide to " hunt for hides" there are two basic schools of thought. Either use the lightest most frangible bullet you can, on the theory that the bullet won' t exit and you are left with a relatively small entrance wound. And the second school says nearly the exact opposite. Use a FMJ that won' t expand at all but punches only a caliber size hole in....and a caliber size hole out. Needless to say when it works.....all is well. But it doesn' t always work. Sometimes the frangible exits anyway and you end up with huge wounds and worthless hides. And conversely sometimes the FMJ will catch a big bone on the way out and blow a huge exit as well.

But the .243 works extremely well on " varmint" class game....and " deer" class game. The important part is to select the correct bullet.....and good marksmanship!