I have always heard not to run reduced loads in magnum type cartridges, anything over about 70 grains of powder, so I never have. I can absolutely see how in a wildcat cartridge or when using an uncommon/unknown bullet and powder combo, how, in the name of caution, you start lower than normal when doing a work up, and end up having this problem. Scary to think about, glad the guy is relatively ok.
My father had an old cock-on-close rifle in .308, as he was chambering a round, the firing pin dropped before he closed the bolt and fired out of battery....the bolt handle sheared off in my fathers hand, the bolt body flew about 20 feet back over his shoulder, and the rifle smacked him so hard in the chest it knocked him to the ground and knocked the wind out of him. I was 8 years old watching him, and though he had been shot he dropped so fast. He was ok after he caught his breath, but he had Winchester 748 powder kernals embedded in his face, hands, neck and arms, he would have likely lost his eyes if he wasn't wearing glasses, they had burn marks all over them. His entire right chest ended up with a black, blue, purple and green bruise about the size around of a soccer ball, and he had a cracked rib. I thought I had just watched my dad die after it happened....that has always been on my mind when handloading, so I do my best to stay safe, but I know that bad stuff can still happen to any of us, so I also try to stay on the lords good side ha ha.
This is likely unrelated to this guys intentions/circumstances, but I have always figured if I want less recoil, still load up to pressure, but just use a lighter bullet. If you want a light recoiling load in a 300 win mag, load some 125ish grain bullets at railgun speed, and recoil will be pleasant compared to a 200+ at 2900 or so. I have a Remington semi auto 30-06 that my son may use to hunt timber with when he gets older, I plan on loading up some 124 hammer hunters in it, recoil should be mild and will still be great for sub-400 yard deer and elk.
Back on subject, I'm glad the guy is ok. Reminders like this are always good for us to periodically read about. Any of us are just a single mistake away from turning a beautiful and precise piece of machinery into a handheld frag grenade with no time delay.