Well my source dated 2001 is quoting 28.4KM with 3OF29, 30.5KM with 3OF59 and 37KM with 3OF60 (which seems to be RAP)
The reason for this improvement in range is said to be improved aerodynamics.
My information is from "Russias Arms 2001-2002" which basically comes from the makers of the weapons and equipment. It has separate entries for the different 152mm guns and ammo and describes the 30F59 HeFRAG round in the 2S5 and towed 2A36 guns as having a range of 30.5km.
The only round with a range of 37km in the book is the 30F43 round from the 203mm 2S7 gun and the range is given and 37.4km.
Was your source Russian/Soviet, or was it a western estimate... in my experience western estimates change depending upon the agenda... ie all Soviet artillery has enormous range... we need new guns/ammo, or we just got new guns and their stuff is terribly short ranged in comparison...
The case for the 2S5 shown on the previous page is reasonably longer than projectile. Does it also seem to be of a larger diameter than the charges for 2S19?
That image is interesting because most of the pictures I have of 152mm propellent charges look like the ones at the top of this page, not the brass cased shell on the previous page.
Ofc a common trope on this forum has been "export/published figures are all wrong, the Russian equivalents have magically massively better performance across the board!".
That is occasionally true, but far from always.
It is a Russian law that where possible exported items don't have the same performance as domestic products... exceptions were certainly made in the last decade and a half where the Su-30MKI was clearly superior to the Su-27s and Su-27SMs in Russian AF service, but as a general rule it holds true.
What is your "source"? Russia's arms catalog? Or somebody who is quoting it?
RA is based on makers figures and I find more useful than even the best estimate/guess from Janes.