The only "data" I can find regarding the use of Herco with heavy 44 special +P is in John Taffin's article Reloading the 20th Century 44s.
In the heavy handloads section Taffin lists loads with the 429421 with Unique going to 9.5 grains and 2400 going to 18 grains! Brian Pearce's article, which includes pressure categories, states that 8.5 grains of Unique is under 22kPSI and 17 grains of 2400 is under 25kPSI with the same bullet. In Handloader 236 by Pearce 8.5 grains of Herco is listed as a max with the 255 grain RCBS Keith bullet at under 22kPSI, which is odd since the same table has Unique going to 8.5 grains and Herco is a slower burn rate.
Taffin says nothing of pressure, or OAL, and leaves out a lot of what I would consider essential information. He also is doing this in Colts which I understand are not really up to the job of the 25kPSI loads. This I find concerning.
So I take his suggested loads of Herco (10 to 11 grains with 429421) with a lot of suspicion and have been unable to find any good corroborating load data to indicate they are safe. I have found some I am concerned about...as the 2003 Alliant load manual lists 44 magnum with a 240 grain lead bullet at 12.5 grains Herco seated to 1.6 OAL an 34kPSI! Does shedding only 1.5 grains drop the pressure to ~25kPSI? The 44 special at 12kPSI and 246 grain LRN is only 7.7 grains! So about half way between those is 10 grains. And this is with bullets underweight of the 255-260 grain 429421?
Does anyone have an old load manual that corroborates this suggestion, like 44 mag data using herco with a deep seated 250 grain Lead bullet (1.550-1.650 OAL) and pressure figures? Does anybody care to plug the info into quickload to get a max load at 25kPSI?
Herco I speculate is the ideal powder for these applications being bulky and the right approximate burn rate. No other powder available to me in that burn rate zone (between Unique and Blue Dot) is as bulky.