Here's what I would do. Shoot the gun using a MEASURED VOLUMETRIC charge of your choice black powder depending on your personal criteria, i.e. heavy charge for higher velocity or moderate charge for accuracy. Your call. Stick to the thresholds for your specific rifle - if it says don't go over 120gr loose powder, that should be your volumetric max. Get your load tuned where you want it, then measure that volumetric charge by weight. Might even weigh 3-5 different volumetric charges to get an average weight. That should tighten up your groups a little, but don't expect magic. It would also be a good idea to weigh your min and max volumetric charges so you know what your weight range will be, and DO NOT exceed that. I personally don't find that shooting max loads yields acceptable accuracy, but opinions vary.
Keep in mind that different brands of powder will have different WEIGHTs by volume. Never assume that a weighed 65gr charge of Pyrodex RS is = a weighed 65gr charge of 777. The same goes for granule size. 65 weighed grains of FFg DOES NOT = 65 weighed grains of FFFg. If you're using FFg, 120gr is about the max you can go before you're just blowing unburned powder out the barrel. FFFg burns faster, so a 150gr charge may actually get a complete burn before the bullet leaves the barrel, so pressure is higher and more risky to work with. Even if they're the same brand they still burn differently and should be treated accordingly.
All these things to consider is the reason that BP manufacturers only recommend measuring by volume. Whatever your reason for wanting the utmost in accuracy, just be careful what you do, learn your min/max charge weights and stay inside that range. Also keep in mind that it's just a muzzleloader. It was designed to kill big game at relatively short ranges where extreme accuracy is not critical. It wasn't designed to be sub-MOA, and most BP guns would never reach that goal no matter the care you take in choosing and measuring components. Regardless, good luck, good hunting, and let us know if you try it and how much better your groups are when you compare volumetric vs weighed charges :up: