A lot of today's gun twist rate are designed to shoot an saboted bullet with a 100gr by volume charge with a saboted bullet weighing in the ballpark of 250gr. This is an "average" load for most shooters and this "average" produces the most consistent accuracy for the the average shooter/hunter. With lead conical bullets they are typically a lot heavier and typically need attention to powder charge to achieve the best accuracy. Same can be said when shooting a say a 300gr Barnes spritzer. It's a heavier than "normal" bullet and powder charge to produce desired results could take some trigger time. A lot don't have the time,money, or desire to find the sweat spot. Nothing wrong with that as life is supper busy and complicated and the time needed to find this sweat spot could be used elsewhere. Also lead used in today's bullets is not of the quality it was 20+ years ago. Also the demand is a fraction of what it 20+ years ago so quality control is not where it used to be. This produces varying weights and heavy spots that cause the each shots to vary. Also several of the older guns designed to shoot conical at a faster twist rate were actually a .504 caliber. So with what is considered today as a"factory" gun just simply want shoot lead conicals without some real bench time. How deep the rifling is cut into the barrel also has a great deal to do with how a gun will shoot lead. Not to mention each gun is different, although may look the same they are different.
Now that that's said you can not compare the factory guns and bullets most shoot with specialty/custom guns and/or bullets. Example, the Knight 45cal 1:20 twist was designed to specifically shoot heavier than normal 45cal bullets. A specialty gun shooting custom bullets is not a fair comparison. That's like saying a professional shooter with $50000 a year worth of sponsor money shooting a $2000 custom muzzleloader with their own hand molded 605gr hard cast bullet with 38.5gr weighed Swiss at the NMLRA 1000 yard match is on the same same as a factory worker who get 3 days to hunt muzzleloader with a CVA Wolf shooting 2 pyrodex pellets and a 245 power belt. Although similar not a fair comparison
So in short, today's muzzleloader are by design made to shoot a saboted bullet within a particular weight range within a particular load range to achieve an acceptable degree of accuracy. Yes some can shoot a lead conical but it takes time, money, bench time, and some luck
So if you want to shoot lead then there are 2 choices - spend money and put in the bench time or find some of the few that produce lead loads like powerbelt or traditions in a sabot. For me I'll stick with the design of my factory guns, side locks get lead and inline gets a saboted copper or brass bullet. Why fight the design? Spend the time hunting and enjoy life