Barrel length can only add to MV if there is high enough energy left in the expanding air to make use of the extra length. For sub 12 ft.lbs about 55 cm is optimal. Beyond this length the air in the barrel has not enough energy to overcome the friction and the extra length will actually slow the pellet down. I am talking from experience as I have all three lengths, Carbine, 41cm and 56cm for my HW 100 and have tuned the rifle to all three lengths. The gain from carbine to 41cm is quite substantial and less so from 41cm to 56 cm.
A.G