Could have also been an underload. A lot of handloaders have experienced "detonation" instead of "deflagration" in the load when the powder fills only a small portion of the cartridge chamber. I saw a 0.5 gr. Bullseye load blow the cylinder out of a perfectly good Super Blackhawk.
They make inert fillers just to prevent this irritating little phenomenon.
My standard load for .45 ACP is 7.5 gr Unique, and it's not a hot load for the .45. Without filler a 6mm load could have detonated.
I can't think of any of the IMRs that would have a small enough load to detonate. Even 4895 would fill it pretty full. That's my standard powder in the .303, it fills to just below the shoulder, and 4320 and 4350 fill up the whole case.
mameluke, why would I want to do that? ;-) I can always re-barrel. Lord knows I have enough extra parts around here . . . and besides, that would mean I'd have to buy another set of dies! Too many wacko calibers around here as it is (ask me about .455 Webley!)