Not trying to be combative...but I don't think you can overkill a deer or any other animal unless it gets to a point to where you destroy meat. You cant kill a deer deader than dead...dead is dead.
Not saying to get a Barrett .50 cal BMG & hunt deer with it...yes that is way too much & would prob blow it nearly in half.:barf: I do think that what folks are meaning is that the .300 Win Mag is way more than is actually needed, but it isnt overkill. If you compare it, you can reliably take deer w/ a .243, 30/30,etc... The .300 Win Mag definately far exceeds thoses calibers in all ballistics.
I have a 7mm Rem Mag & it has ballistics that are very, very close to the .300 Win Mag. I don't typically deer hunt with it though. I reserve it for long range hunting, deer hunting near the reclaimed strip mines of my south eastern KY home (when I am on Military leave). Additionally, when I retire from the USMC in 6 years I will be getting into elk hunting...the 7mm Rem Mag will be my go to weapon for this quarry.
The 30.06 I own is my go to weapon 90% of the time. The recoil is lighter, the weapon is lighter, & it has more than adequate ballistics to drop a deer. Nothing wrong w/ the .300 & I know many people that use it for deer hunting. With that beast (.300)...I think any type/weight of round will work for deer, even Managed Recoil type rounds. Sorry for ranting on, I'm new on here & just really glad I found this forum!