Having never killed an elk with a muzzleloader I do not have in the field experience to offer you. I do know the Knight Extreme as I owned one for a while and shot a lot of bullets out of it. A very nice accurate rifle. I traded it off for a White .451 Model 97. Since I had a Knight Disc rifle to fall back on.

I would think three pellets of Triple Seven (if accurate), would be a great load. I you want to try others, and if legal... try BlackHorn 209. My Extreme did real well accuracy wise around 110 grains of Triple Seven loose powder. Sabotloader shoots a lot of Knight Rifles and uses Triple Seven with some outstanding accuracy. Others shoot BlackHorn in their Knights with amazing accuracy. But I will state, I had good luck shooting Pyrodex RS pellets as well in my Knight Rifles. While I would go with loose powder, you shoot what is most accurate and what you feel comfortable with.

I would suggest a 250 grain or bigger bullet. I base this on range work I have conducted, and from reading experiences of people I trust a great deal, that have hunted elk. The 300 would be a great choice IMO for what it is worth.

I have shot the Blood Line bullets in my Knight Disc and they were very accurate. I suspect they would be accurate in your rifle, but one can never tell. Some of the better constructed bullets out there are Barnes, Nosler, and Lehigh (Bloodline), bullets. On my range they have held up to a lot of abuse.

The main thing is use a well constructed bullet, pushing it to as fast as accuracy will allow. Be able to place that bullet and don't try to make it do more then you are able. Shoot for the main organs and you should be fine. Good luck on your elk hunt. I hope you shoot a big one. Then we can ask you about how well the bullet you selected worked for you.