Laws of physics say it was a fast powder.
Ken - interesting idea. Care to show photos? Must have been firefirming many many cases.
I think I did 120 or so in one session. Pretty efficient once everything is in place.
I now use Nosler brass (generally get it once fired), so fireforming is a thing of the past. But, I did it for both the 25-06 and the 280 AI versions. No longer have the 25-06 AI. Saves a bunch of bullets, powder, range time, etc.
The pipe was about a 4 or 5 foot section of 4" PVC. On the one end I put reducing bushings to get it down to a size just above the barrel diameter. Then I also wrapped painters tape on the barrel, to keep the soot and crud from building up (stainless barrel, no sight)
On the other end of the pipe, I used a 4x3 reducing elbow, and then a vertical piece of 3" pipe, running down into a large plastic trash can with a hole cut in the lid for the pipe. I put about 6" or so of water in the can to help further catch the small dust. The pipe can't actually be under the water, or the force of the first shot will just produce a geyser, and blow the lid off the can. Clamped the rig to a wood work bench with pipe straps. Also, important to keep a clothes dryer running on air cycle or something to pull fresh air into the room. Interesting note- even with that, the process did trigger a CO detector upstairs. If I ever did it again, I'd do it in a garage with a window or door partially opened. Even in a city environment, the actual sound produced would sound no different than some kind of impact tool or nail gun.