Need more horsepower

[onelastshot]

My favorite long range deer rifle is a 270 WSM. Plenty of knockdown power and has the reach that you're looking for.
A 130 ballistic tip traveling at almost 3300 ft/s; it's got the ability to reach out for the 300-400 yard shots. In spite of it's magnum status I find it comfortable to shoot and not much different in terms of felt recoil than my Sako 270 and noticeably less kick than a 300 WSM.

For Elk I'd simply change over and use 150 grain bullets.

 

[pogson]

with a decent bullet, good shot placement you have a 400+ mule deer and 300+ Elk gun

a 150 at 2900 has about 1550fp at 350yds and 1300fp+ at 450yd

I agree. Perhaps, instead of a new cartridge the OP should invest in a laser rangefinder. Reloading is important too. For such long shots you want a very high BC bullet which expands well at the terminal velocity and the highest safe muzzle-velocity that has sufficient accuracy. A 165 or 180 BTSP should do. For elk, you might be a bit better off with a 190 BTSP. I'd carry two or three different rounds for hunting: 180RN for walking around when a close shot might present, 165BTSP for most other situations and 180 or 190 BTSP for when you know you need something extra at long range. Changing loads and bullets has almost the same effect as rechambering/rebarrelling at a much lower cost.