When I test casings for reloading data, I normally mark each casing with a letter and number, then mark the projectiles (when zinc, you can stamp them with the same letter/number combo as the casing). I keep a log of the launcher I use with each, barrel length, or anything else that changes between rounds. 5 rounds fired from each barrel shoud give you a good result. You can then do a drawing of the target area, indicating point of impact of each round fired, and break down each to it's chrono reading. This will give you the best results for comparing barrel length, and should indicate whether 3" makes a great difference. With the high/low pressure system, I doubt you will notice any, as the main thrust is imparted at ignition, and not during barrel travel. With a rifle, and pistol, you are effecting range and speed with barrel length. With the 40mm LV, all the power is generated when fired, and goes downhill from there. Happy testing, GROG