Boy Semi, that is hard to swallow getting groups like that at 50 and 75 yds then have it expand that much at 100 yds. I would have tried another group at 100 to see if it wasn't operator error.(even though I know you are a 'crack shot'). Could have been your eyes were getting a little tired or you got smoke in them.
I'm hoping to get out and shoot my 54 this weekend with PRBs. I am going to start with 80 gr of FFg and work my way up. I had to order a cleaning jag and need to get that in before I start shooting.