Originally Posted by
longbow
I had a similar experience many years ago with my .45-70 converted Siamese Mauser using a Hornady load of IMR4227 under a Lyman 457125. This load was particularly HOT! though it was out of the book. Being a frugal kinda guy I looked for economical powders and this load of IMR4227 was the lightest powder charge to achieve the same sort of velocities as IMR4320... not quite as high but awfully close.
The first time I pulled the trigger the gun kind of magically appeared about 6" behind where my shoulder had been, of course taking body parts with it. The recoil was a nasty sharp jab that really, really hurt! I guess no surprise with a powder that fast under a heavy boolit.
While they shot okay and I had no barrel leading, there was melted lead around the case mouths. I had never seen that before and not since. I have to suspect that the powder was so fast that there was gas cutting at the boolit base as it left the cartridge but then the boolit sealed the bore.
In any case, very similar situation to yours so I suspect the fast powder is the culprit even if you aren't over pressure.
Something else to look for too is if you can recover any boolits more or less intact, check the lube grooves for collapse. I have had ACWW Lyman 429421's collapse at the large single lube groove using book loads and no pressure signs. Again, I suspect the powder burn rate was too fast so short sharp acceleration curve and yielding alloy.
I'm sure there are others here that have more experience and maybe better explanations but that's my take from what I have seen.
Longbow