The original question asks about a "300 mag."
I am a bit of a stickeler when it comes to naming cartridges. Or using the correct name for a cartridge.

I heard a story about a non-resident alien hunter who was up here in Alberta with and outfitter and missed a really big white-tail. The outfitter suggested they go to a nearby range and shoot the clients gun (a "300 mag."). The gun would not group worth a darn. The outfitter asked if the ammo the fellow was using was the same he had sighted in with. "No". said the client" I sighted in with .300 weatherby mag. ammo then could not find any more so I bought these .300 winchester mag. shells to hunt up here with.

There are several ".300 mag." cartridges now adays and they are not interchangable.

If your gun is a .300 win. mag. then always state that and be sure you use that ammo in it. Same goes for people saying they shoot a 7mm. That could be one of several 7mms.

Any of the big .300 mags. would be great elk cartridges IF you use the proper bullets and IF you can hit where you are aiming with them and IF the hit is in a vital area.

Robin