I got into a similar discussion over on
www.longrangehunting.com and let me tell you, some of those guys are using higher math than me!
The simplest explanation I received is that the bullet spins around it's center of form in the barrel, then transitions to spinning around it's center of gravity after it exits the barrel. These two points are always a little different due to the inexact science of bullet construction.
The longer the bullet, the longer it takes to make the transition. During this transition period, the bullet is not stable and bullet paths are somewhat divergent. After stabilization occurs, the bullet path "straightens out", and the bullet paths are much less divergent.
The bullets do not curve back towards the center point, but they do not have to in order to have smaller MOA groups at longer distance. If we were talking strictly inches and not MOA, group sizes do continue to grow as distance increases. Sometimes not much after stabilization occurs, but they do get bigger.
I did have a guy claim that he saw a heavy barrel rifle that would shoot 3 inch groups at 100 yds but at 300 yds the groups were only 1 inch (yes inches, not MOA). I stopped short of calling him a liar but said that there was no way I would believe that unless I saw it--my diplomatic way of calling him an idiot.
I hope this explanation contributes usefully to this topic. I'm not an expert on exterior ballistics but it made sense when explained to me.
