Shot placement. Remember the scene in BHD where the CAG guys took cape buffalo with 5.56 from M733's (Colt Commandos IRL, yes I know most of them were actually M4's in the movie) ? That part actually did happen. Maybe not exactly the way it's portrayed in the movie, but nonetheless, it did happen. However, there are far better choices for elk. Personally, I'd want at least a heavy load in .30-'06 or .308, I'd prefer 7mm Rem Mag. It's doable, but not sure about the legality and there are better choices. If you had a rifle laying around in good shape that fires 8mm Mauser (such as one of the never used or lightly used Yugo M48A's, you could use a heavy 8mm load to do the job. The problem is, most 8mm in the US is downloaded for surplus rifles. Europeans have hotter 8mm loads which are capable of taking virtually all big game on the planet easily aside from elephants and cape buffalo (and even then, it's a possibility). If you could get ammo like that in the US that hasn't been downloaded, you would have a cartridge ballistically similar to .30-'06, but throwing a bigger, heavier bullet,which will give a penetration advantage. I wouldn't recommend 7.62x54R because it can be hard to find decent hunting loads in that caliber and there's plenty of other available calibers that are much more powerful. 7.62x54R is in the same league as .303 British, 7.7mm Japanese, 7.5mm Swiss Service, etc in that they're all roughly .30 caliber cartridges (most in that list are actually .311) but all are rather underpowered compared to rounds of similar size in similar calibers.