I need a 400 yard deer gun. Here’s my options......

[Skyvalhunter]

$$ must not be a problem

[opdinkslayer]

A couple things I’ve noticed in all the suggestions. First I 100% agree with practice & trigger time but nobody mentioned making sure to have a good trigger to practice behind. A good trigger,reasonably accurate rifle & practice will make 400 yds very doable provided you can control the excitement factor & realize the environmental factors of that particular moment. A 400 Yard shot with buck fever on a blustery day is far more difficult to execute.

[theleo]

243 has plenty of energy for deer past 500 even with 95gr bullets.  Get some trigger time and a decent scope and it will do fine.

A 165 gr. .308 (.30-'06) has approximately 50% more kinetic energy at 400 yards than a .95 gr. .243, with a bullet like a Winchester silver tip.

Kinetic energy doesn’t kill stuff.

That's about as naive a statement as I've seen on here.  If a hunter is going to shoot an animal at 400 yards, he had better have adequate energy to compensate for less than perfect shot placement.  If the round he is shooting has minimal energy to do the job, then he had better be very selective in the shots he takes...............and most hunters are NOT.  And, the .243 falls into the category of minimal kinetic energy to kill a deer at 400 yards with perfect shot placement.

Read this:

Kinetic energy, the ability to do work (or in this case damage), is the most common measure of killing power for rifle bullets.

https://www.chuckhawks.com/rifle_bullet_killing_power.htm

Cut and paste from the article:
Energy and killing power

"Kinetic energy, the ability to do work (or in this case damage), is the most common measure of killing power for rifle bullets. And it is, in fact, a reasonable indicator. But it is by no means the only factor, or even the most important factor. Energy gives us an idea of how much power there is to initiate things like bullet expansion and penetration, but does not guarantee that they will occur.

It is generally recommended that a small bore (.24-.32 caliber) rifle bullet suitable for medium size (CXP2 class) game be carrying about 800 ft. lbs. of kinetic energy when it hits. Energy is greatest at the muzzle, and diminishes as the bullet loses velocity. When the velocity reaches zero, so does the energy. But long before that the bullet has fallen below the recommended level of energy for reliably killing deer size animals. So the practical hunting range of any cartridge is ultimately limited by how much energy remains. (It is also limited by other factors, for example trajectory, but that is another subject.)

As an example, let's take a .30-30 rifle firing a Federal factory load with a 150 grain bullet at a muzzle velocity of 2,390 fps. At the muzzle that bullet carries 1,900 ft. lbs. of energy. At 100 yards the energy has fallen to 1,355 ft. lbs. At 200 yards the energy is down to 945 ft. lbs. At 300 yards the energy has fallen to only 650 ft. lbs., which is below our 800 ft. lb. minimum. The velocity, by the way, is down to 1400 fps at 300 yards. One could conclude that the .30-30 is about a 200+ yard deer cartridge, based on its energy, and one would be right."

800ft/lb is the recommended amount of energy, so by that it could also be said needing anymore energy means you should practice more so you stop taking terrible shots and relying on the rifle for your ineptitude. Also, Nosler Trophy Grade 243 with 90gr Accubonds carries 923ft/lb at 400 yards. So even according to the article, the 243 is completely up to the job so long as you do some practice.   

[HawkCreek]

Whatever means you use to measure a guns power and put it to paper it's still a bit of an arbitrary comparison. The old .44-40 papers put at less than 700 foot pounds at the muzzle yet how many big game animals did it kill in its day? Is it comparable to a .30-06 or .243, I'd say we'd all agree that it is not. But that doesn't mean that it's unsuitable for deer hunting. And now I'm way off topic...

[konradcountry]

The problem is the wind.

Even if you are a consistent shot at the range you can't reliably dope the wind in the field.

This is where I see the 243 being a problem at 400 yards. A gust of wind pushes your bullet and now you are tracking all night in western wa blackberries. Not fun.

I'd rather have a bigger hole from the 06 in case I am off.

But with said I get the appeal of going light. That is where the smaller calibers shine.

Sell them both and get a 7-08.

[doubletall]

Practice has been mentioned a few times and the only thing I will add is to practice in field positions as often as posible.  I can shoot prone using my pack as a rest almost as well as shooting off a bench.  Where I've found the need for more practice is situations where the rolling terrain or vegetation won't allow a prone position.  Shooting from a kneeling position, even with shooting sticks is something I need to practice a lot more. 

[birddogdad]

all said in thread

larger bullet/ energy
high bc/wind is the enemy
trigger time
great optics
practice
look for a flat shooting round

know your limits... 400yds is a chip shot when you get after it!

[Magnum_Willys]

Practice has been mentioned a few times and the only thing I will add is to practice in field positions as often as posible.  I can shoot prone using my pack as a rest almost as well as shooting off a bench.  Where I've found the need for more practice is situations where the rolling terrain or vegetation won't allow a prone position.  Shooting from a kneeling position, even with shooting sticks is something I need to practice a lot more.

Pack works great with a rear rest - a roll of t.paper in a baggie works well for that and light too.  If you need to get off the ground there's some examples out there for bracing buttstock with shooting sticks in addition to shooting off them - I need to practice that but if I can't get prone my range drops way below 1000 right now.