I have not heard of the .300 magnum having vicious recoil. Perhaps this is portrayed by bloggers, as you say, killercrow. I don't know about it.
Some .300's do have quite nasty, snappy sharp recoil. A Mark 5 WTBY I once shot was rather sharp & nasty, but lacked the overall whack of larger
bores. The Mark 5 stock didn't fit me well, and the recoil speed was high. Coupled with the sharp crack of the muzzleblast, I assume added to the
recoil.
Of course, felt is quite subjective. A fellow at the Barnet Club sold me a brand new, unfired ZKK600 .375 H&H barrel for my rifle (.358 NMag.), for $100.00. He had bought
the ZKK6002 .375H&H just for the action. This was mid 1970's. He had Ian Dingwall pull the barrel, replace it with a .510br. & chamber up a .505 Barnes Supreme.
I happened to be at the range when the short heavy set Hungarian was there with the rifle and he offered me a couple shots. The rifle, with it's new stock and
4X Weaver couldn't have weighted over 8 pounds. The fellow had fired about 8 or 9 rounds off the bags and was sighting it in at 50 yards. I fired one shot, off-
hand, gripping the forend quite hard, and was braced for a belt. Well, that thing spun me around like a weather-cock and I ended up back feel sliding on the concrete
back behind the yellow shooting line a full step back behind where I had been standing. That one kicked.
125gr. IMR3031 with 600 gr. Barnes bullet @ 2,650fps. I was only 190 pounds in those days and VERY fit/strong. I went up to the club house and drank coffee for an
hour. Then went back to my bench, shooting my .375H&H. It felt like a .222, even though it hurt to pull the butt onto my chest/shoulder pocket.
That's the hardest I've been hit. Felt like getting hit in the shoulder by a Volkswagen bus. That's the analogy I made, back then.
So - yeah, recoil is quite subjective - to the person and physical traits of the gun.
If in a 20 pound rifle, that .505 wouldn't have kicked more than my .375, which I rather enjoyed shooting, due to it's soft, easily managed recoil. That rifle weighed 11 pounds.
However, a 20 pound 4 bore does kick, like this one. The light 12 dram load produced roughly 200 pounds of recoil. A .458 Win, Mag, which is rather nasty, produces slightly less than 60.
https://www.youtube.com/watch?v=MDYtxxRU_cY