outdoorsmen
what do you think the odds are in getting constant pressure/compression on the powder when seating the bullet? from what i understand that's important with BH209, right?
With progressive powder the compression aspect of the bullet is very important also, as well as the weight of the bullet. You will need to have compression on the bullet and the sabot will need to seal the barrel to prevent the escape of gases. So using an easy loading sabot may not be the way to go - you will need a tight one.
Most often when we load we find a seating pressure that feels good and we tend to stay right around that presure everytime you seat a bullet - so I think that portion of the formula will work itself out.
Weight of the bullet and the amount of pressure to push the load down the barrel are going to be important because either one of these can change the performance of the powder.