Originally Posted by
Larry Gibson
goodsteel
..... I think that the reason the RPM theory exists, is because all the standard rifles out there, with a very few exceptions, damage a soft lead boolit before it gets out of the barrel. ..........At the pressures and speeds we are shooting, lead begins to act like silly putty, and any small misalignment can really have a detrimental effect on it as it is squirted out of the barrel. ie, what get's to the target is not the nice concentric boolit that dropped on the bench.
You are for the most part quite correct. The problem is one of casting a balanced (concentric certainly is a large part of that) bullet, keeping balanced through the sizing/seating GC, keeping the bullet balanced during loading and keeping it balances during shooting i.e. shooting or the internal phase of ballistics. If the bullet is not balanced on muzzle exit the physics of the external ballistic phase take over. At some point of RPM the centrifugal force makes the bullet depart from the line of flight on a tangent or it makes the bullet (the most often occurrence to happen first) begin a helical arc around the line of flight which slowly increases non linearly in group dispersion as the range increases. That is the reason why most find the accuracy of their cast bullet loads decrease at some point.
That point is not a limit as some pundits keep wanting to believe. I have pointed out many times that the RPM threshold can be pushed upwards by controlling the amount of unbalancing that occurs to the bullet during acceleration (shooting). We can control that by the design of the cast bullet, the alloy used, the fit to leade, bore and groove diameter and by using slower burning powders that offer consistent ignition and burning. Basically the more balanced we can keep the bullet during casting, loading and acceleration the higher we can push the RPM threshold upwards.
BTW, several of the anti RPM threshold pundits lay claim to marvelous accuracy at high velocity (well above the RPM threshold) using the very factory and milsurp rifles you say it can't be done with???? Sure would like to see how they do it...........
Larry Gibson