You've made me re-think a bit my feelings about gas cutting. I will agree in the main about friction etc. being the culprit, but must also think that gas cutting does have a somewhat deleterious effect on the stabilization of the bullet as it makes its way down the barrel.
At the end of the day, what all this analyzing, theorizing, and posturing points to is the base of the bullet is key to its successful flight. If the base isn't flat and square, and sufficiently large (either through bumping out or proper sizing) to seal the bore, it simply won't fly true - it is indeed the steering end of the bullet. Of course other things factor in too, but this speaks to the heart of the matter. At high(er) velocities the gas check serves in several capacities, but maintaining the integrity of the bullet's bases is chief among them.
As I mentioned before, Charlie Dell's experiments conducted by observing captured plain base bullets shows conclusively (to me anyway) that it isn't hot gases that wreck a soft PB bullet's base, rather not-yet-burned powder granules having a sandblasting effect. My own experimenting with PB soft cast bullets in my couple of .32-40 High Wall target guns (one of which is Pope #53), breech seating with over-groove diameter bullets so there's no question about starting with a perfectly sealed bore, show a marked increase in accuracy when I interpose a thick felt wad behind it (best) or a wax wad (even better). I think (but I could be wrong) that the wads simply protect the bullet bases from the sandblasting. (Bullets are lubed sufficiently as I enjoy a heavy lube star on the muzzles even without the wax wads, and the lube I use is a simple 75/25 mix of beeswax and Alox, not terribly high tech.) Besides, I need a wad in the case mouth to keep myself from spilling powder as I insert the thing into the chamber behind the pre-seated bullet!
Of course, the above is really only germane in discussing low velocity soft PB cast bullets at modest velocities. I have little personal knowledge of high velocity (over 2000fps) protocols. I have no desire for such - I simply have no need at this time although I might change my tune if my insanely big stash of jacketed bullets for .22CF's dries up - my academic cast bullet interests lie firmly in the 800-1800fps range, rifle and pistol.
