I was wondering why some rifles seem to be much more accurate after minimizing the bullets distance to the rifling and others do very well at magazine length and big jumps? I imagined as the powder burns the brass expands from back to front assuring that every bullet leaving for a fractional instant is always concentric. Meaning in the first fractional second the back .010" of the bullet neck is fully and tightly expanded to the necked chamber centering every bullet "before it even leaves" to whatever neck chamber the machinest made. In short every bullet is aimed directly at the rifling by the earliest part of the gas expansion. Of course if the brass is thicker on one side this violates the perfect alignment of every bullet with the rifling even before it leaves the brass theory, but good brass is pretty even walled. So assuming your brass is decent or turned (for thickness variations, not diameter or concentricity) does it all come down to every bullet jumps either aligned with the rifling if the machinest got the original neck / bore alignment concentricity within .00001 or takes off at a built in angle because he machined it .0001 off? Then I thought, any amount of off-set in original machining concentricity is an angle which is multiplied by the distance of the jump. Having run a lathe there are lots of tricks a great CNC machine or machinest can do to make it more accurate but it will never be perfect if you go enough decimal places. Repeating myself cause I like too, pretend its a conclusion: Does it all boil down to how good the original machining was that determines if your particular rifle is a don't much care jumper or a gotta have jammer? Is the rifle's tolerance for jump a direct indicator of machining accuracy even if we can't measure .00001 (add whatever 0s you like or point out I used too many for dramatic effect) concenticity? Is a good jumper a better barrel at heart and the machinest you want to buy beers for? Then, even if you jam a miss-aligned barrel does it completely solve the problem or does the bullet still feel the angle via loading when it leaves the brass and carry forward a 10% vibration even though you cured 90% by jamming? Newbie to forum, probably should have started with something like whats your favorite color brass, Sorry, had fun thinking about the why today. PS: Of course a great rifling "lead in" helps all ills, but probably doesn't change the original question.