I was watching an old WWII movie, which showed the rifled insides of a cannon barrel. A similar view (of a smaller gun) is found at the beginning of one (or more!) of the James Bond movies. This rifling all seems to be of a *fixed* pitch -- i.e., a helix of constant twist angle. But if you think about it, a bullet starts at velocity zero and angular momentum zero, so *ideally*, this rifling should start out relatively shallow, with slightly more twist angle as the bullet travels further down the barrel. On the other hand, the bullet is also travelling *faster* at the end of the barrel, than at the breech of the barrel, so the twist could actually be less at the end, since it may already have achieved the correct angular momentum. So what's the ideal rifling -- by "ideal" here I will choose that rifling that maximizes the energy transferred to the bullet, while still achieving a given angular momentum ? We have to transfer a certain amount of energy from linear motion to rotating motion, and I presume that there is a certain cost associated with this transfer. Obviously, the ratio of the moment of inertia to the mass of the bullet should enter into the calculation at some point. In order to maximize the power transfer, there needs to be a kind of *impedance match* of the bullet to the rifling pitch. https://en.wikipedia.org/wiki/Rifling "In some cases, rifling will have twist rates that increase down the length of the barrel, called a *gain twist* or *progressive twist*".