Having owned (note past tense ) a Lyman Mustang, and having had the chance to compare notes with a few other owners I would have to say the Mustang would not rank among the top rifles when it comes to accuracy.
I did finally find a load (Barnes 250 gr Expander behind 100 gr T-7 ffg) that would produce acceptable groups at 100 yards (3-4"), but certainly not a rifle/load combination I would use at 200 yards.
In all my years of shooting I have only owned a couple muzzleloaders I would be comfortable at making 200+ yard shots with, and then only if I had time to be 100% certain of the distance. A T/C Omega and a Rem. 700ML/SS. Both of those rifles would give close toMOA accuracy with careful loading. The Remington was the best for accuracy having produced a few 1/2 MOA groups with the best loads, and handled a wide variety of powders & bullets well.