All,
The original load the guy tried was 100 grains of AR 2208, which should have given him about 2150-2200 fps, not that underloaded, but not a heavy load either. No fillers were used at all, that I'm aware of.
I've personally seen the chamber cast, and the throat is ridiculously long, we are only theorising that the long throat had something to do with it as well. (Also Bertram cases were used in the original loading, not that that should have a bearing either)
It was quite a surprise when the 600 grn Woodleigh could be dropped into the throat, a sized case chambered and no resistance felt at all. This is where the short neck on a 500 J is a downside, whereas the much nicer neck on a 505 might have permitted him to seat the bullet out to be just shy of the lands.
My own 500 J was built using a Clymer reamer that Pro-Cal had at the time, it has a much shorter throat, I can seat a 535 out to touch the lands if I needed to.

DC