I call BS, a 200% overload is a triple charge
200% of something is double. In your case you used a 150% charge. You start at 100% and go up from there; 0% is nothing, not the starting point. If I had said "200% more than normal" yes it would mean a 3x normal charge but I said a "200% charge" which means 2x normal. Considering I could barely fit the double charge into the hull I doubt I could get a triple charge to even fit with any room for a wad or shot.
In truth they weren't true 200% loads... only 190% and 138% loads.
I have loaded 1oz slugs over 80 grains (max recommended is 42 grains) of Blue Dot with CCI magnum primers. Produces a 4' fireball out the end but only kicks about as much as a 3-1/2" super magnum buckshot (Remington.)
I have loaded 9 pellet 00 buckshot over 25 grains of Red Dot (max recommended is 18 grains I believe but it's not rated for a load of this weight regardless) and it made the base convex but fired fine. Recoil was painful but nothing broke.
I have put 200+ shells (mixed target, buckshot, and slugs) through the 870 since these tests and nothing has gone wrong at all.
I started at safe loads of Blue Dot and just kept working up. Once the primers started to invert I stopped. It was an experiment more than anything and there where no signs of over pressure on the primers or hulls until around 150% max recommended Blue Dot. It's not like I woke up one day and just tossed in 190% max powder to see what would happen.
Also since the 190% loads where with a magnum powder there isn't as much of a pressure spike at the beginning. If I had tried 190% Red Dot charges (a much faster powder) it would be more likely to damage something.
The Red Dot 138% load was an accident. I meant to try a ~125% load (had already tried 110% and 120% loads) but used the wrong bushing and loaded them with 140% instead.
*Edit*
Got my Red Dot numbers wrong. Was going from memory; checked the files and it was only a 140% load (originally posted 177%). Still though a 140% load of fast powder was worse (for my shoulder and the hull) than a 190% load of magnum powder.
I've personally blown up(on purpose) a shotgun with a simple 50% overcharge of HS-6.
Was it a Remington 870? If not than it's not really a fair comparison. I have seen old single shot shotgun barrels crack from having a patch in the barrel.
IMO, you're stupid if you think 15K PSI safer to toy with than 60K PSI; both will f**k you just as bad.
But 60k psi in a barrel/chamber that can only handle 125% of that vs. 15k psi in a barrel that can handle 300% of that is a huge difference.
I would not want to be anywhere near a gas cylinder filled to 1200 psi that is only rated for 750 psi but I have no issues standing next to a gas cylinder rated for 450 psi that is only filled to 150 psi.
Shotgun shells and load data have to be made to be safe to fire in any off the shelf shotgun of any quality. If a shotgun is greatly overbuilt compared to the worst case scenario then of course it can handle higher pressures. As I said even the hulls and primers weren't showing any signs of firing a hot load until I hit the 150% mark with a magnum powder.
MidwayUSA has a video of a 12ga H&R single shot being fired with a 20ga shell lodged in the forcing cone; the action popped open and the barrel bulged.
This was with one of the strongest 12ga actions available too.
20ga in front of a 12ga in an 870, no failure:
http://www.youtube.com/watch?v=laX3H08TuMk
2" of mud in the barrel of 870, no failure:
http://www.youtube.com/watch?v=EWl1Mu1tywM
.50 BMG from a 12 gauge (H&R?):
http://www.youtube.com/watch?v=sZaK7D4XVo8&feature=fvwrel
If you've actually tried it any of your claimed practices, I have no doubt you'll be picking bits of barrel out of your face...
Been there, done that, still shooting from the same barrel
Though I have never fired a 20ga in front of a 12ga personally. I have watched my uncle do it (I checked the load and there really was a 20ga in front) and he claims he had done it dozens of times over the years.