RE: .243 for whitetails
A 100 gr 243 bullet has the same sectional density as a 130 gr 270, and factory loads in these weights have the same velocity. On game like deer, it will penetrate just as well as the 270. Deer shoulders, ribs, hearts and lungs are not a problem. And no deer will eever know the difference of .027" bullet diameter between a 243 and 270.
Factory soft point ammos, like CoreLokts or PowerPoints, perform awesome out to 250 yds, and they are cheap. Exit wounds are usually about 1", and most deer run about 25-40 yds with lots of blood. 25% of the time, they fall on the spot.
On deer, inside of 150 yds, I can't tell a difference between my 30-06, 308, 7 mag, or 243.