There are finer grades of powder sold, and the finer sizes are probably the left overs from making the larger ones. The 6F and 7F are used today in the firecracker industry, I am told. If there is a 5F powder, I have not heard of its use.
As you can see from the responses, most people tend to use what they first learned to use as Priming powder years ago, and continue to use it out of habit. There is a small difference in burn rates between 3F and 4F powders. However, the coarser grade( including 2F) tend to absorb moisture more slowly, and make a good choice for use while hunting.
BP is coated in graphite( hence the "g" on the end of the F Grade) to slow absorption, to prevent clumping, and to improve flow through the manufacturing process. The added benefit of coating the granules in graphite is that it gives a substance that allows static electricity to pass quickly through and around the outer surface of the granules, so you don't build up enough heat to ignite the powder with static electricity. That makes Black Powder much safer during transportation.
Try both FFFg and FFFFg powders for priming, and see if you can notice any difference in ignition times. The machines say there is a difference, but most people can't see or hear it. More important, you won't move your front sight off the target in the slight delay it takes to ignite the FFFg powder, compared to the FFFFg powder.
The first couple of times I went deer hunting with my flintlock, I carried both horns with me into the field. The large horn has FFg and the small one has FFFFg priming powder. That got very old, after several days. I have since made up individual powder tubes, and no longer take my priming horn out with me at all. I use FFg or FFFg powder to prime, depending on what powder is going down the barrel.
The large horn only goes with me these days when I am using my shotgun to shoot birds, where I may have to take 4-6 shots before returning to my car. But these decisions, like those of other members who have given their opinions and experience here, is based more on what I consider my personal CONVENIENCE, than anything else.
There is nothing wrong with modifying what you do on the range, for your own convenience in the field. Just understand the reason WHY we use ONE powder in the field, and some of us still use 2 powders at the range. Oh, If I am dressing up for a public parade, or demonstration, I wear and use the two horns, along with all my other gear. Those are the only times I carry my throwing knife, AND my throwing Tomahawk, for instance. The tourists and kids like all the stuff. I even break out a hand made Bowie Knife to carry on those occasions. Without suspenders, my pants would be down around my ankles, of course! :nono: :blah: :rotf: :hatsoff: