I'm in the opposite direction of most posters here. Fify caliber slug! Who needs expansion? Get something with a fairly flat nose and don't worry too much about velocity. If you get it going 1300 to 1350 fps it's going to pentrate well and probably exit to boot.
I'm in CO too and understand the regs you are working with. I think some of the stuff suggested above in involves sabots, etc., which as you and I know are a no no in CO if you are hunting in the ML season.
BTW, if you are hunting with your encore during a non ml season, you can use the scopes, pellets, sabots, etc. and be legal.
I once tried some 450 grain slugs molded from a LEE .515 mold designed for the .50-70 Govt round. The guy who gave them to me had sized them down to .504. Talk about a hammer on both ends!!!! Once they ran out, I never went that way again because I didn't have the correct sizer. Now though LEE will make you a custom push through sizer of your specified size for about $20 bucks. May be time to revisit those slugs.
As far as powder goes, there is no doubt in my mind that more genuine black powder is sold in this country than all the substitutes combined. The idea that the substitutes are superior is pretty well sold to the in line community, but the fact is that real BP will ignite more easily and contrary to popular notion it cleans up as easily as any of the subs with warm or even cold water!
BTW, since you are in CO and anywhere near the Denver area, I have a few 370 grain Maxi balls you could try if you like.