Got this from shotgunreport.com:
"Dram equivalency" is one of those smoke screens the shell makers
intentionally use to keep you from knowing something. It was originally
meant as a measure of black powder back in the old days. I don't know
whether it was 1/8 of an ounce or 1/16 of an ounce of black powder. I've
seen both numbers given for the weight of a dram. Perhaps someone more familiar with black powder shooting than I am would know.
When smokeless powder arrived, makers looked for some way to equate the velocity of a smokeless powder load to that of a black powder load so
that the shooters of the day would have a reference point. They settled
on dram equivalency. A load of 1-1/8 oz of shot pushed by 2-3/4 drams of
black powder would have a muzzle velocity of 1145 feet per second. A
load of 1-1/8 oz of shot propelled by 3 drams would go 1200 fps. Even
though the new smokeless powder didn't measure the same way, they used the black powder dram equivalency measurement as a secret code for velocity of the new shells.
The problem is that the manufacturers never changed even after smokeless powder became the standard. I believe that they didn't want to get into a velocity competition. It would have been just as easy to mark the shell "1200 fps" instead of "3 dram", but then the next power crazed
shooter would want to buy the competitors shells that went 1210 fps
because they just had to be "better". And so on and so on. To avoid
this, I believe that the manufacturers have intentionally kept the
shooters in the dark by using the dram equivalency rating.
Today, that wall is starting to crack and some makers are entering the
speed race by printing the muzzle velocities on the package. That's not
always clear either. Some of the European makers even hype their
American market numbers by using a rating of a pure muzzle velocity,
rather than the standard American measurement of velocity at 3 feet.
This adds about 150 fps to the European numbers. Every nation's proof
house has their own way of measuring velocity and they don't all give
the same numbers.
It's almost the same confusion with "high brass" and "low brass". The
brass shell head was originally put there for case support as well as
for the rim needed to hold the shell in the chamber and provide the
extractor some grab. With modern plastic hulls, the metal isn't needed
for case head support any more. It's just for show now, but some
shooters believe that the longer the brass, the more powerful the shell.
So, when makers want to emphasize that their particular shell is really,
really hot, they use a head with lots of brass to make it look
impressive.
The "handicap" rating of a shell is even more arcane. American-style
trap shooters often feel that they want a more powerful shell when
shooting from long handicap yardage. Current ATA rules permit a maximum velocity of 1290 fps for a 1-1/8 oz load. The shell makers wanted the shooter to know that he was getting a shell that was more than 3 drams (1200 fps), but didn't want to tell him exactly how much more. Hence they made up the name "handicap". I think that these loads are closer to 1250 fps than 1290 fps, but I've not chronographed any recently so that's just a guess. If you are shooting handicap trap, then you ought to buy "handicap trap" loads, right?
All this confusion would be so simple to solve if all the makers simply
stamped the velocity of the shell on the box. It's all part of the
"Cleaner! Whiter! Brighter!" soapsuds marketing syndrome where consumer ignorance is good for business.