Charlie I start seeing the obturation problems and PC loss only with 8-9 BHN alloys at 2100 fps for lube groove bullets and 1800 for smooth sided bullets in the 30" twist. 1-1-98 alloy. 98% lead. You are shooting 90% lead. Your alloy is so hard compared to what I was talking about. I am nowhere near the rpm threshold, therefore I can shoot butter soft lead at high speeds compared to small bore guys.
At 35k PSI I can't even get the lube grooves to deform with 5-5-90, let alone the stronger driving bands. This is why the PC stays intact - the outward obturation forces are very low with the hard alloy and low pressures involved with shooting a big 30 cal case at only 2300 fps. You need your bullet hardness to bite the riflings and resist skidding at the high RPMs and you don't need bullet hardness to counter obturation forces AKA lube groove collapse.
My 1-1-98 alloy at 2100 fps, recovered down range where it slowed below expansion speeds and loaded to just before the obturation abrasion point of the PC shows the lube groove has just about obturated away to nothing and the lead from the inside lube groove shank is starting to engrave the riflings. The only hint of the lube groove is that the PC there is much cleaner and less blackened from the carbon fouling in the barrel. If you load it any higher than that amount of obturation/ lube groove collapse the base driving band will be naked of powder coat and the barrel will get immense whitish PC fouling.
The 1-1-98 lube-groove bullet at 2100 fps shows the base is forming a pretty decent cupped hollow base from the pressure and the friction dragging the driving bands backwards. the 1-1-98 slick sided bullet shows the hollow base at 1800 fps, right where the accuracy starts dropping off. I imagine that up to a certain point of pressure the bullet engraves and scoots out the barrel. As you keep upping the pressure, the bullet forms a shallow hollow base from the pressure, and the friction pressure at the base of the bullet goes up exponentially. Increase pressure 5% and suddenly with the hollow base formation on the bullet, friction goes up 10% and your PC abrades away, just like it would if you jammed the bullet between your tire tread and slammed the brakes and skidded it along asphalt with a ton of weight on it. The obturation forces can literally measure in thousands of pounds when you are talking about 35k PSI chamber pressures.
Imagine on the slick sided bullet when the pressure forms the flat base into a hollow base how hard the lead is getting squished outwards and its only path of flow is lengthening the bearing surface. If you cut off the barrel at the 1" mark of a 35k PSI 9mm luger while shooting soft lead you would find the base on the recovered bullet mushroomed out to around .40 caliber as it exited the muzzle. Do the same test with a hard bullet and the base would hardly deform. The same thing can be shown on a bottleneck rifle with a jacketed bullet - if you cut the barrel at the point of peak pressure, the bullet base will mushroom when it exits the crown. On the lube-groove bullet mold the obturation forces on the base band are transferred directly to the smallest diameter of the lube groove, and once the lube groove shank diameter grows enough, or meets enough hydraulic resistance from lube, then the base will finally start to form the hollow base shape.
If the lube groove is empty (or mostly empty with a thin skim of tumble lube) it keeps the base nice and flat up to some certain pressure level then suddenly the lube groove bottoms out into the riflings and the bullet's only direction it can obturate is to form a hollow base and lengthen bearing surface(which is one reason why PC bullets can outshoot lubed bullets at higher velocities and softer lead - the naked lube groove that slowly collapses under pressure, the other reason being the hydraulic pressure of the lube in the rapidly shrinking lube groove acts as a sizing die between the bullet and bore, making the bullet undersized and allowing lube to blow past the bullet, which leaves a naked undersized bullet for the last few inches of barrel travel).
If you go only a little bit harder than 8 BHN - 35k PSI - 2100 fps - the obturation abrasion stops, or a little lube on the 8-9 BHN and the obturation abrasion stops.
