A 300 mag, 30-06, 308, and a 30-30 all shoot basically the same bullets. The 300 hits a 600 yards at about the same speed as the 30-06 does at 500 yards, the 308 at 400 yards and the 30-30 at 100 yards. I'd have no problem using any of those rounds at those ranges and would expect the same results from them all.
It is all about understanding range limitations, bullet construction and placement.
And by the way, I'd use a 243 for elk. Not my 1st choice, but I'd use it.
This is for you Brian.