Brent,
  One of the big myths in BP shooting is that you must fill the case with powder. You must not have an air space! Yet if you read the loading articles in Shooting & Fishing there are literally dozens of references to just the opposite being done.

  Also I've done some experimenting along this line and nothing disastrous happened. I will say tho that I didn't get better accuracy than I would get with a full case under some compression.

  With the .28/30 I'm playing with now I'm breechseating and the best powder charge only fills the case about 2/3rds full. On top of that I put a 1/16" cork wad. There must be pretty close to 3/4" of air space in there and I've fired a coupla hundred rounds with nothing unusual happening. I was getting terrible results doing it the "excepted" way until I started thinking outside the box! 

  A very big part of the problem with shooting BP is that a lot of these myths are what is holding us back from getting the same results as the old timers got. Due to expensive or unavailable material on the subject we have been re-inventing the wheel these past 20 yrs. or so.

PETE