If you get a quality rifle made and you practice, 1/2 mile shooting will become EASY down the road. I like the 300 RUM because it is hard to outgrow for a big game hunting rifle yet its perfectly at home at any realistic ranges.
If you get a new custom rifle made and you get practiced at longer ranges then its best to have the rifle chambered in something that you will not outgrow.
As far as using lesser calibers on elk, obviously they work but keep in mind that elk are very big, heavy boned, heavy muscled animals with an incredible will to live. Moose are much larger then elk but I would say an elk is easily twice as hard to put on the ground. Let me rephrase that, an elk can take twice the pounding of a moose before he gives up!!!
I have seen mortally wounded elk RUN for over a mile and never show any sign that they were even wounded in any way and that was after being hit with 300 Wby. That elk was lost for a time. The only reason I know it was mortally wounded is it was found by another group of hunters the following week.
With the short mags work, CERTAINLY.
Can elk be wounded with larger calibers, CERTAINLY.
Is there ever a bad side to using a larger then needed cartridge, NEVER, as long as the rifle is designed properly for the use at hand and with shooting comfort in mind.
We need to remember that the larger expansion ratio of a given chambering, the more efficently a good muzzle brake will reduce felt recoil. A 300 RUM in an 8 lb rifle with a good muzzle brake such as a Holland QD or my APS Painkiller will have felt recoil about like a 243 Win in same weight rifle. In comparision, a 7mm or 300 WSM in the same exact speced rifle will often have MORE felt recoil then the much larger 300 RUM.
Why, simple fact that the big 300 has much more muzzle gas volume and pressure which dramatically increases the efficency of the muzzle brake to reduce recoil.
The lighter the rifle the more dramatically a good muzzle brake can reduce felt recoil simply because a lighter rifle is easier for the muzzle brake to slow down from recoil and a lighter rifle has less momentum once its started in motion.
A heavier rifle will often times MOVE the shooter more. REcoil velocity will be slower then a lighter rifle but the muzzle brake will also have a harder time slowing the rearward movement of the rifle so while the felt recoil of a heavier rifle will not be severe, the rifle WILL move you more then a lighter rifle with a good muzzle brake and same load. It takes much more rifle weight to tame recoil to the same level as a good muzzle brake.
It is a fine line however, go to light and you loose shootability, if you go to heavy and its to heavy to pack in the hills.
In my opinion, its far better to have more chambering then NEEDED to get the job done then use a chambering that SHOULD be adequate........ There is only dead, there is no DEADER or DEADEST.
As humans we are the weak link in long range shooting, we are not perfect, in fact rarely perfect, get the largest, most impressive ballistically performing chambering you can handle accurately and you will never be sorry. Other then some extra powder, a 300 RUM is nearly exactly the same price to load for as a 7mm or 300 WSM, dies are the same, rifle cost will be extremely similiar.
Just my opinion concerning lesser round on elk size game at long range. THey can be easily killed by lesser rounds but I have seen alot of them get lost using lesser rounds as well. Also seen them lost with larger rifles when hit wrong so it all comes down to shot placement in the end but no body should be scared away from a large rifle because of recoil any more, we have very good ways to eliminate recoil.