With just the 209 primer!!!  Well we realized that the moment of ignition our chamber, the space between the saboted bullet and the breech plug was growing as the bullet began to move. Now right off we found we needed a very tight fitting sabot.  If you load smokeless powder load with a loose or light fitting sabot 99 out 100 times it will not fire.  Smokeless powder needs confinement and pressure to burn at optimum rates. I'm sure all of you have lit a pile of smokeless and watched it fizzle. We started loading bigger and bigger loads of slower powder. We found a sweet spot at about  20 % above what was a standard load. We had to compensate for the ever expanding chamber. This led us into a new direction, duplex loads !!!! Ahhhh!!! Duplex loads??? Yes, when some of the guys started using heavy loads of slower powder they noticed two pressure spikes milliseconds apart. Some actually got bulged barrels at 1-/12" to 2-1/2" in front of the saboted bullet, does that distance sound familiar??? Yeah well that's also where the second pressure spike occurred. This is where duplex get comes in.  Remember slow powders need long barrels and tight twist rifling to do their thing. What happened is for a given load of say 75 grains by weight 15 grains of that would be 4759 and the bulk 60 grains would be 4198. What this does is bring the slow powder up to optimum pressure quicker, this made for great accuracy and great velocities. No any company that recently came out with ammo in standard calibers shooting anywhere from 100 to 200 FPS without causing over pressure signs in shooting these loads???  The guy that put all this research together was a good friend of mine and I know he was selling all his info to a reputable manufacturer.  Unfortunately he passed away before I knew any more about it.
Shortly after he passed superamunition was released. I think you know who I'm talking about. Now the 30-40 krag has a long throat for the 220 gr bullet.