Hodgdon releases none of this information to the public. So it would seem to follow that they do not release IMR or Winchester information either. I would also believe the other powder manufacturers do the same. So any numbers such as KJ/KG, would seem to have to be derived from post manufacturer testing.
The folks at QuickLoad must have tested their lots of powders, for their numbers.
The QuickLOAD program compiler gets his default values from powder manufacturers. Most companies won't provide these data to the public, but some provide it on their websites and elsewhere. For example, Nammo Vihtavuori Oy gives both bulk density and specific energy values on all its rifle powders on its website:
https://www.vihtavuori.com/powders/n100-powders/
https://www.vihtavuori.com/powders/n500-powders/
European manufacturers seem much more willing to provide this information than their US equivalents - Norma Precision (Bofors manufactured), Nitrochemie Wimmins a.g. (Reload Swiss and some Alliant grades), Vihtavuori all do so. Norma also shows the nitroglycerin content of each of its grades which is pretty unique. (All Norma powders are double-based.) The shooting journalists who research and write Wolfe Publishing's
Propellant Profiles features in
Handloader magazine usually obtain these types of data from the manufacturers or US importers.
FWIW, QL's predictions in my use of both IMR and H4350 have proven pretty close to actual, in one recent case spot on for the Hodgdon version. Both are rated at 3,760 J/g in the program and they don't appear to be far out at that. Purely an opinion!
I somewhere have a feature in a very old handloading tome about this aspect (ie the manufacturers' use of 'closed bomb' testing to ascertain powder energy characteristics) which uses IMR rifle powders as its example. This was written back in Dupont Corporation days when IMR powders were manufactured in the US. One interesting point the article made was that all IMR rifle powders had the same energy value (which was given in BThU - British Thermal Units) such is the length of time since publication, and that the burning rate as opposed to energy levels was controlled through kernel size and the diameter of the central longitudinal hole as well as use of deterrent coatings. That may not apply now as IMR powders have been developed and manufactured for the Hodgdon Powder Co. by General Dynamics Valleyfield, in Valleyfield, Quebec Province, Canada for many years now.
I suspect that many energy values are an average or 'nominal' figure as actuals will vary perhaps considerably by production lot as noted in earlier posts. 'Canister powders' for handloaders are meant to be much more consistent than the bulk lots supplied to ammunition manufacturers. It used to be said that supplies from different lots were blended to achieve this, but I suspect that nowadays a lot that gives close enough results to the nominal values on testing will be diverted to the handloading supply side of the business. Vihtavuori has long prided itself on making exceptionally consistent powders, so it may not have to do this, or maybe not so often as its competitors.