The old test was, wait till you have snow on the ground and increase your charge grains until you see some unburned powder in the snow. 90 grains is plenty. The short barrel will cause you to lose some velosity due to potentially reduced loads and I wonder just how much bullet stability you are really getting out of the slow twist and the short barrel?? I'm not sure whether to suggest a 495 ball and a thin .010 patch or a 490 ball with a thick .017 patch to give you the best rifleing contact. Maybe others have a good suggestion, but I'd try both.