Hi John:
Properly chastised by Clive, I got into my copy of Doug Knight's book.
The MV for AP was about 2600 fps for a six pound projectile and for APDS almost 4100 for a 2 1/2 pound projectile. Nearly 1.6 times faster.
Getting back to the original question regarding the propellant charge; I'm now certain that the charge remained the same and the higher velocity was all due to the lighter weight of the projectile. Lt Stewart is vindicated as he only spoke of velocity not propellant... my apologies if I appeared to cast aspersions on his memory.
On the "felt" recoil with the longer barrel, it is interesting to note that the US 57mm M1 had the same length barrel as the Mk IV gun yet they never fitted a muzzle brake. More reading required!
Mike