You will have to excuse me for sticking my nose in because I have never shot an elk, I have been shooting muzzleloaders since the 70s though....

For elk, I'd go with either a 300 gr Barnes all copper (the original) or the
300 gr Shockwave...As far as powder charge, 100-120 grains should be plenty as long as shots are within 125-150 yards...

Good Luck, sounds like a ton of fun....