About 2F for prime. I did, the odd time when I ran out of 4F, use 2F for priming in the .44. I noticed more fouling around the pan from the use of 2F, over 4F. This might have simply been
due to the less exuberance of the 2F, in comparison and not blowing the fouling away. I found I had to check the vent after priming, to ensure it was not blocked with fouling from the
last shot. If I didn't check it, I got a flash in the pan now and then, (not every time) and when looking at the vent, there was a "piece" of hard fouling blocking the vent.
This had NEVER happened when using 4F for priming.