I spoke with Graham Greener at the SCI convention. He strongly cautions anyone against using the BP action for anything and he repeats anything other than standard shotgun cartridges. So follow the lead of its inventor and use a really strong falling block for the 12 Gauge from Hell. No Greeners!

Sarge-That's why I am building a bigger version of the GP,
for the full pressure loads, for 12GA FH,
with a front locking lug feature.Lugs are four times the size of the lugs on my Ruger 77.The breech block will be
2 inches longer and wider.Action sides will be about 3 times thicker.Now you can use original GP, our brass, as
our strong brass cases shortened about an in with a little higher pressures.These BMG derived cases will add 5000 psi
capability with their extra strength alone.And a heavy
rifle type barrel, as using rifle loadings will get a pressure curve that can split the front of thin shotgun style barrels.In no way are we recommending using the monster in GP barrels, or anywhere near full loads with heavy barreled GPs..Ed.

I relegated my Greener GP to a 12 Bore Paradox Gun project.It will use a 12 GAFH short round and won't churn up much more pressure than a std 12 GA. I still can't figure out how they made that cut in the back of the action for the breech block hindge pin. It could be easily done today by EDM but back then?-Rob

Please clarify, are you talking about the 14ga shotguns with the 3-prong firing pin? If you are there's lots of people risking their lives every weekend. Just at my home range alone there's at least 2 guys that have converted these to 50-90. They whack away at a 24" 440-yard gong and they do quite well.

Rob-I wonder if they used a small milling arbor supported on
both ends, with a small cutter fitted after arbor was in the holes.And the action will be perfect for your Paradox project.

Gonzo-Yes those are the same actions like Rob and I have.
Mine had regular single firing pin, and lucky Rob had the
three prong deal to change over.And those 50-70 guys aren't in any danger.With steel of about 80k tensile strength
and it is probably higher,figuring cross section,
divided by two(standard engineering for shock loads), the
sides of action can hold about 20,000 lbs thrust, that is transmitted by the breech block to its support in the rear
of the action.And a 50-90 loaded to 35,000 has 5500 lbs
of thrust.Ed.

According to Ross Seyfried in a Rifle article a year or so ago, this action was used for 303 Brit and a number of other centerfire rounds. Seems like it would handle more than the 11,000 or so psi of a shotgun round. That being said, a bore round on a shotgun case must be a gamble as far as pressures. I understand shotgun cases are much more unpredictable for pressure changes based on component changes. Still, there are some documented loads for paradox guns and round ball loads that would stay in shotgun pressure ranges. bob

bobc:
Regardless of what Seyfried may have said or implied, the Greener GP action was used for shotguns, not centre-fire chamberings. While it is very similar to the Martini-Enfield action developed for the .303 cartridge, the latter has significantly more 'meat' behind the breech-block hinge (the GP is 'cut away' somewhat). This would become significant when higher pressures than the standard Brit 12-bore cartridge were expected. I strongly suspect that the GP would handle higher pressures than generated by a standard 12-bore, but probably not the pressures proposed by some of the enthusiasts on this thread! (hence their quest for stronger actions).

As has been pointed out, the real issue is back-thrust on the breech-block, not pressure per se. A .50-90 at 35,000 psi may well only generate 5,500 lbs of backthrust, but a 12-gauge at that pressure will generate 18,000 lbs of back-thrust due to the bigger cartridge head area. (less a percentage for the strength of the brass, of course). Buggered if I'd be volunteering to be the first to stand behind it, but I DO want to watch!!

ED- Thought of the milling arbor approach but how could you advance the tool into the work? You'd wind up taking too big or too small a cut. I thought about a single point tool with a mini adjustable head approach but it would have to be incredibly small. The Greener GP action appears to be milled rather than cast so thats out. EDM would work, but they did'nt have that technology. I'd love to know how they did that. Maybe it's obvious, but not to me! By the way if Graham Greener designed the GP action he probably have to be 140 yrs old or more. Martini Henry's were around in the 1880's. Frankly, I'd bet they can easily stand 40Kpsi. I've seen more than a few 14 Ga Greener GP's converted to 444 Marlins!-Rob

Marrakai-I been studying mine and I think they will do
25,000 with our Bmg derived cases, for good safety margin, as you would reduce the thrust by what area thr sides of the case takes and the load the sides can hold can hold which little over a third
at those pressures.So 25000 times base area of .51 sg in
equals 12500 times 2/3 gives 8300 lbs thrust.Action shear figures out to 20,000 lbs with its 2 to 1 margin of safety,
in using half the yeild strength of the metal in figuring shear.Damn shame they didn't make sides twice as thick, and
reciever ring thicker to match..It would be wildcatters
heaven.

Rob--In milling that I wonder if they had a shaper tool that
cut the metal out of that recess??On a short stroke machine.
I'd like to know myself.Ed