RE: Lets be honest with powder charges.
I believe 200 or 300 fps is very signifigant in a muzzle loader. The heaviest charge I shoot is 130 grains of 3F BP in my MK85. I shoot 120 grains of 3F Triple 7 in my long range hunter. Both rifles shoot extremely accurate. IF they did not shoot accurately I would get rid of them. I personally want the extra velocity in my MZ's. But then recoil is not an issue for my hunting rifles. I want all the performance I can get. I will admit that I shoot 80 grains of 3F BP in my flinter. It is my fun gun, and extra velocity is not necessary(and the rifle probably will not burn much more).
The trick (IMO) with heavy loads is consistency. Consistency is the reason for inaccuracy out of heavy loads. The larger the load-the more consistent you need to be in your loading, cleaning techniques(while shooting), bullet depth, and most importantly your ignition. Ignition is the key. You must have the ability to burn the powder in your barrel.
Your burn rate(again IMO) is dependant on the weather,your powder, and your ignition system. Some rifles just can not burn 150 grains of powder when the humidity is high, but the rifle can burn 150 when the humidity is low. Therefore accuracy wil suffer because of the amount of powder burn. I also believe rate of burn is an issue also(which is also affected by weather). I know this is splitting hairs, but I think it is very important in MZ's.
There is an easy way to prove this, which I have done before.Take a 130 grain load of 3f BP and shoot in a high humidity rain or snow storm(hunting conditions), then go out and shoot the same load, bullet, and rifle in a summer like low humidity, and warm weather situation. You will get a differnt POI, and accuracy will probably be different. However if you lower your load to 70 or up to 100 grains you will see a lot less difference in POI. This IMO is due to amout of powder burn.
The old timer's who taught me how to shoot flint lock'stold me to sight in, and shoot my rifles with the snow test(remember 2F flinter's will have a slower ignition). It was the only way they knew how to verify how much powder their rifle's were burning. Tom.