I really like guns and I like variety, so I would never deride or make light of what caliber rifle people want to use for deer. However the truth is that a .243 Win or 6 mm Rem, both have more than enough power and flatness of trajectory, to kill any deer that ever lived out 300 yards. However I don't believe you can overkill something, you can only kill it.
You don't specify which 7 mm, but the 139/140 grain .287 is a good performing bullet. A man armed with a 7x57 or 7 mm-08 loaded with a 140 grain bullet could kill most anything. In my 7 mm Remington magnums and the one 7 mm Wea I had, I really liked the 160 grain Speer flat base. It is a really good looking bullet and loaded to around 2900 fps accurate and devastating out to as far as it is practical to shoot with a hunting rifle.
I've only ever owned three .300 Win Mags a Ruger 77, a custom built one on a Mauser 98 action and the one I currently own a Stoeger J9. The only one I killed a couple of deer with was the Ruger 77. I used a 150 grain bullet, that was back in the mid or late eighties and I believe it was a Nosler ballistic tip. You can see several of the rounds in the above picture I posted loaded with ballistic tips, green is .308 caliber, yellow is .277, brown is .264 and blue is .257. I thought they were neat back then, what with the caliber color coding and the tips that didn't deform in the magazine well under recoil.
Anyway back to you question a 150 or 180 grain bullet in the .300 Win Mag is pretty much the only choices for deer. I would not go lighter and I don't see why you need heavier. The better 150s will hang together OK on deer without being overly destructive.
I've shot quite a few deer with a .30-06 a various 150s loaded to around 2900 fps. It is absolutely devastating, my longest shot with .30-06 from my pre-war Winchester Model 70 was on a doe at 340 yards. When I pulled the trigger you could hear the bullet screaming across the field and the whomp of the 150 grain bullet hitting the rib cage, she went down like a bolt of lightening struck her, fun stuff.
If you use a good 160 grain 7 mm bullet or a good 180 grain .308 caliber bullet( Nosler Partition, Swift Sirroco, etc) and you do your part, you can kill anything in America and most of what is in the rest of the world.
Shot placement is like the four rules of safety, whether it is hunting or self defense shot placement can never be over mentioned.
There is my .300 Win Mag Stoeger J9. I've never killed anything with it, but loaded with a good 150 or 180 grain bullet, it will flip any hog or deer I've ever seen on its ass.
