I have used 4F almost exclusively for decades due to the quantity I’ve historically had on hand, and the approximate 2000 shots/pound utilization rate. On occasions I have used 3F to prime, usually with the occasional follow-up when hunting, wet/high humidity conditions, or my larger caliber/locks rifles/shotguns. Having done some testing, I found it difficult to perceive a difference in ignition time due to the size difference between the two granulations. On the other hand, good supply or not, I would likely stick with 4F with a few of my high precision rifles due to the very fast and precise quantities dispersed from my 3/4 gr. pan chargers, and the tendency for 4F to be less susceptible to shifting location (with movement/hunting conditions)) in the pan compared to the somewhat “slippery” 3F. These factors do seem to make a difference with my consistent lock times/ignition……Perhaps, odd-man out, but just my experience.