Now that Hodgdon has the distributorship for Winchester Ball Powders, they show IDENTICAL loads with IDENTICAL pressure and velocity for W231 and HP38. Obviously they are just running one set of tests to serve for both.
But in 1994, Lyman Pistol and Revolver 2nd ed. showed significant differences in the two, presumably shot with the same bullets out of the same P&V barrels. They did not pair them up in a lot of calibers but in those they did,
HP38 was in every case the "faster." Some examples are:
25ACP 50 gr FMJ maximum pressure with 1.1 gr HP38, 1.4 gr 231
.32 H&R 85 JHP, max CUP with 3.5 HP38, 4.0 231
.38 Super 121 gr cast, max CUP with 5.9 HP38, 6.5 231.
.38 Special 158 cast, max CUP with 4.3 HP38, 4.9 231 (only 6 fps apart)
.38 +P 158 cast, max CUP with 4.5 HP38, 5.2 231.
That is from 10 to 15% more 231 for the same maximum pressure as HP38, not counting the .25. Way more than the 3-5% commonly given as acceptable lot-to-lot variation.
The question is, which has changed for them to be listed IDENTICAL now?
One cynical poster has said there is no change, they spec it all as HP38 and just put 231 labels on what they get orders for. Nobody will see a difference without a supply of older powder and a chronograph.
My old metal can of 231 is only about 5% slower than my big jug of HP38. But that matters to me because I load .38 Special on a machine with fixed powder bushings. I can't readily adjust for the difference.