Jbar: Yeah, 60 grains...
Must be a Civil War reenactor thing? We use 60 gr. blank charges strait down the barrel (no ramming) in our reenactments, so we've all figured that's also good for shooting... Personally, I use a Traditions Hawkin in .50, and I recall the manual saying something along the lines of 70 max...
Are you sure you're not overloading?