I found this graph showing the relationship between muzzle velocity and barrel lenght in calibres. It suggests that the pay-off is in muzzle velocity is quite good up to about 75 calibres in lenght, but decreases rapidly from there on. Little or nothing seems to be gained from increasing the barrel lenght beyond 100 calibres, but then few tank guns was more than 70 calibres long. I was just wondering if anybody could comment on the accuracy of this graph ? And if the relationship between lenght and muzzle velocity is constant, not influenced by the weight of the projectile etc. ?