Thanks Quercus~!
The big shot & egg example actually clarified the headspacing issue for me. I wasn't seeing it too clearly. Now I understand why, when I look at the instructions, the Army also suggested the use of a 30" dowel.
I'm gonna resize the dowel tonight before tomorow's test. I'm not worried about my own safety since, bodily, I'll be behind cover, using a string. Gotta peak the camera out to see it all though.
Since mossberg already has done this, I know it can be done. As far as the experimenting, I'm "reinventing the wheel" here, but can't find test data for it.
Can't wait for the launch!