Ok, got it sorted. Bullets were getting squeezed down by the brass, causing blow-by, gas cutting, and acrobatic bullets. So I went through my whole die collection trying to find an expander big enough to do the job. Nothing. After a bit I noticed the Lee rifle PTE die insert in the bottom of my universal tool head can and mic'd it....471" OD. So I turned it down to .457" on my drill press with a file, shaped a nice bell shank, tapered the point, polished it, and fixed my neck tension while still being able to use my PTE measure.
After testing two other powders I've come to the conclusion that Quickload is simply off the mark by about 80fps and a LOT of pressure with Hodgdon Longshot in this instance. What really got me thinking is a hint of metal flow on the case head, just a slight imprint of the ejector hole and extractor groove, classic warning signs with the .458 that pressures were just at or a little over maximum. I had loaded 14.0 grains, predicted to be about 1000 fps and 26K psi. I got 1080 fps, which would take 16.5 grains to make by prediction, and 35500 psi, which is just over maximum, just like the brass was telling me. My explanation for the 80 fps gain over predictions had been suppressor "bump", which is common enough and I see it regularly. So I worked HS-6 and Herco for a while and the velocities were within single digits of predictions, so it wasn't the suppressor, it was an erroneous prediction. Herco was spot on with Quickload's predictions =/- 2 fps. Problem is, I want to stay around 25K psi and Herco has to run up near max pressure to get 1020. HS-6 does the pressure and speed fine, but has a really high muzzle pressure compared to Herco and Longshot. So I re-ran Longshot in Quickload, taking into account the 80 fps off the mark and came up with 12.7 grains for 927 fps. Shot those over the chrono and actually got 1005 fps with an SD of 1.2 for ten shots, so 78 fps off from the prediction. By adding powder to the prediction, my pressures come out at 28K at 1008 fps. Whatever. The 12.7 grain/1005 fps actual load is quieter than 11.5 grains of Herco at 926 fps, and the action works 100%, rifle feels good, and it shoots like a laser beam. I'm one happy camper now, sore shoulder and all, and that Mastadon is going to be in very, very deep poo....