Actually I prefer a 3-9 or a 2-7 for a muzzleloader. I generally keep em on the lowest setting. The only time I use the higher power is when I'm taking long shots or working up loads. When I'm working up loads I would actually like more power than I have.
Back in the days when I used to buy variable scopes, I made the "discovery" that when hunting I generally had them set at no more than 4X all the time. So now, I usually buy "old" fixed-power scopes, and use either 4X or 6X models as a rule, with 2.5 - 2.75X versions on my short-range guns. At my age, you'll do nearly anything to keep the weight down!! I used to laugh at the old-timers like Jack O'Connor when they wrote about keeping guns light. In those days I lugged an 11.75 pound Win. M70 Super Grade .375 H&H around the mountains of Alaska. No Longer! I don't laugh at featherweight rifles any more!!