barrel length will also be an issue, if your using a spring gun, because 650mm is a long barrel especially for a spring gun. From what I've heard the 6.01/6.03 debate, depends on which company your buying from. company A is supposed to have better 6.01 barrels than 6.03; while company B has better 6.03 barrels than 6.01 barrels. Though I don't have the money to test this theory myself. Your best bet though is to research your gun.