FFFFG powder is generally used for small pistol and priming charges. It is very fine and really not too suitable nor recommended for the larger calibres. As GOLDENLAKEPETE states, he is using FFG powder in his loads.
It is better to use existing paper or plastic shotgun hulls rather than brass ones. The problem is that the brass shells are a bit thinner in wall thickness, and thus do not seal properly when fired with standard 16 guage wads, and you have to buy special oversized wads for the brass shells. 16 guage wads for regular paper or plastic shells are readily available.
I once made up a 12 guage Cooey single shot for Southern Ontario Deer hunting many years ago. The barrel was cut to 24 inches and a front sight, along with a Williams adjustable aperture rear sight was installed. These improved sights worked very good and not only made the gun shoot more accurately, but the improved sights extended the useable range by about 10 more yards or so. One advantage was that you could carry a few standard shotgun shells along for birds or rabbits, and you still had a shotgun that would give a reasonable pattern at 30 yards.
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