The Lyman Shotshell Reloading Manual tables list MVs from 1135 up to 1335 for birdshot loads. All those loads lose most of that velocity within the FIRST 20 YARDS. The Fastest load listed is a load of #2 shot at 1330 MV; its down to 1035 fps. at 20 yds.
When the weight of the pellet decreases( due to smaller diameter in size), More VELOCITY is Lost in that first 20 yards. For instance, a lot of #9 shot fired at 1200 fps MV is down to 820 fps. at 20 yds.
Pellet energy also tells the story: That #2 shot load at 1330 fps. has a pellet energy per Ft. lbs. of 19.09. At 20 yards, its down to only 11.79 ft.lbs. That's a loss of almost 40% of the energy each pellet has at the MUZZLE.
That #9 shot pellet has at that 1200 fps. MV has pellet energy of 2.40 ft.lbs. But, at 20 yds, its down to 1.12 ft. lbs. That is a 53% loss in Ft. lbs. of energy per pellet in that short distance.
The tables don't list data at 10 yards( 30 feet). And, you can't really simply divide difference at 20 yds, by 2 to get an accurate estimation. Those pellets will be going a bit more fast at 10 yards than that kind of math will indicate.
I did look at data listed in a table in the old Greener book, "The Gun and its Development", as it shows reading at 5yds( 15 feet) and in 5 yd increments on out to 60 yds. Unfortunately, it doesn't list values for either #2 shot, nor for #9 shot. They list a #10 shot, which is as close as we can probably come considering how old the data is in that source.
Using the data for #6 shot size, Greener shows a 4 dram( 110 grains) load behind 1 1/4 oz. of #6 shot giving 1199 fps( close to 1200 as you can get) MV at 5 yds. At 10 yds. that is down 1177, a LOSS OF 2% VELOCITY. At 15 yds. its down to 1159, a loss of 4.4% velocity. At 20 yds, its down to 1122 fps., a loss of 6.5% velocity. At 25 yds. the velocity is shown as 1082 fps, a loss of 9.8%. At 30 yds, its down to 1034 fps, a loss of 13.8% velocity. If you plot these numbers on a graph, you will see an increasingly downward curve, that is NOT quite Linear, even tho' this load began just above the Speed of Sound.
This "trombone bell curve" is typical of what you see for Round Balls as they lose velocity after leaving the muzzle. The heavier, larger balls carry more mass, and therefore can be shot over a longer distance, before they begin showing a Lot of drop. But, when they get far enough from the muzzle they drop much faster than do smaller, light pellets, resembling the bell of a Tuba, instead of a Trombone!
There is a footnote on that page in Greener's book that quotes another expert, speaking at a symposium for Gunmakers, in 1896 saying that it appears that velocities are improving since powders are getting better, over the original data taken in 1878.
I mention this just to caution readers about placing too much "faith" in old data. Everything about Everything to do with our sport has seen improvements since the Am. Revolution. What we shoot today is much better in all ways than what the Founding fathers had available in that time. :hatsoff:
