Yes you could be right, but I've seen a stuck projectile in a 9mm semi-auto pistol - primer only, no powder. Took a suprisingly decent whack on the "ram rod" to knock it back out. 9mm is a reasonable bore size considering also that it was only a small pistol primer that did this.

I'll try it in the 458 when I get home tonight. I have to pull apart 16 remaining cast reloads anyway (for powder and load adjustment) and will try one with primer only and see what happens. This will be without crimp, as per the other reloads I tried.

Maybe the last one that got stuck at the range did burn some of the powder - the change in colour and bunching may have been caused by the heat of some of the powder igniting perhaps?