Dave: The ball is spinning at its highest rate of spin just as it leave the muzzle. That centrifugal force will keep the misshapen ball on line to that short target. But the spin begins to slow down just as its forward movement begins to slow. Remember these RBs have a terrible BC! That .490", .50 caliber RB has a BC of .057! Its sheds velocity like a balloon. When the spinning begins to drop down below a certain level, and the forward motion also slow, the misshapened ball now is affected by the air, and winds, and everything else, including the sound barrier. That is why a ball that drives "X's" at 25 yards, can miss that 50 yard target. the effect is even more pronouced at 100 yards.
The word is Obterate, and refers to the ball upsetting and filling, or trying to fill the grooves of the rifling. One of the reasons patch thickness needs to be fairly close to the groove depth is so that the Ball will NOT obturate so much that it loses its round shape entirely.
Shoot a round ball, with a small charge, into a bucket of water( or barrel if you have it), so you can recover the ball without it hitting anything solid. That is how you can see what your loading technique, size of patching, the amount of force used to get it into the muzzle, etc. affect the ball. A good fit will be a ball that shows some weave pattern embossed on its surfaces, some slight flattening caused by the lands, but basically a round ball that still has a round nose, and a round back end. The inertia of a PRB is light enough, assuming the ball is the usual undersized lead ball wrapped in a lubed patch, that the bottom should not be flattened when it is fired. If so, are you using FFFg instead of FFg powder? Try the FFg and see if the ball does not stay more round.
I dealt with all this stuff with my first rifle, a .45 caliber. I began shooting a .445 ball, because that is what someone " TOLD ME " to use. I did not measure the bore, or anything. I had thin, .005" patches. They allowed me to get the ball down the barrel, but they burned holes in them with the FFFg powder charge- which was 45 grains to start. I finally got it to hit sometimes at 25 yards, but had all kinds of flyers at 50 yards. I switched to .440 balls and a .010" patch. That worked much better, the ball was not distorted by pounding it in the muzzle with my short starter, and all of a sudden I started getting better accuracy at 50 yards. I had a chance to fire the gun into a barrel of water, to recover some balls, so I took both my .445 and my .440 balls, some powder, both sized patches, caps, and some cleaning fluid. The patches floated on the surface of the water and were easy to recover. We used some kind of stick to pick the balls out of the bottom, and you could clearly see that the large ball was terribly misshapened just from loading it, even with the .005" patches. The .440 ball, and its .010" patch were both in better shape, and intact. Whwere the weave of the .005 fabric was clearly impressed in the lead, and clearly visible, over a wide flattened surface, the .440 ball showed less weave, and the weave that appeared was less distinct. I think I used 25 grains of FFFg for those tests, but that was more than 30 years ago. I am allowed to be a little fuzzy in my memory about that after all this time has passed, and I had no need to recall it.
I did watch other members of the club struggle with their guns claiming they weren't accurate, etc. when all that was wrong was their choice of loading components. When we convinced them the guns could shoot off a rest, they still could not hit targets, because they didn't know shooting fundamentals, and would not ask for instruction, or take it if offered. It takes all kinds of people. I told the club members when I agreed to be the club's attorney, that If I bought a rifle, I expected them to help me learn how to load and clean it, and then I was expecting them to help me learn how to be a good shot. They all laughed, until I did acquire that .45, and I kept my word. I asked the dumbest questions, but I learned.
! 
