It's made from a .41 magnum case necked down to a .356 caliber bullet and is shooting that bullet at almost 30-30 velocities.
It’s not making that speed in a GP100, not by a long shot. Gary’s data for an 8” Blackhawk is 1815-1850 max for a 158grn pill, a good 500-600fps slower than what can be attained with a Dirty Thirty. Even given a long barrel, there’s simply not enough case capacity there to reach the same performance - the 356 case is giving up about 15grn H2O to the .30-30, even with higher pressure, it’s a gap too wide to close.
I push a 180 to 1900-2000, depending how frisky I want to get, with a Redhawk in .357/44 Bain & Davis, with about 4grn H2O greater case capacity - and a hell of a lot more cylinder wall thickness than a GP100. I cheated my way into a 356 GNR in a Contender, and wouldn’t quite keep up with my 7.5” B&D.
Personally, the 356 GNR is right where I’d want to be in a GP100, and equally, the GP100 right where it should be. All the power it can handle, and not a drop more. Also, personally, I’d shoot it with glee until it rattled loose, and then have it tightened up and start all over.
I’ll confirm, there is a gross misconception above that these operate at substantially higher pressures than .357mag. Quickload does the math for us - we can put a heck of a lot more powder under these bullets without as much pressure. Have to remember - revolver cartridges will reach their pressure peak typically while the bullet base is still within the cylinder, and the larger cases have larger internal volumes - we can put proportionately that much more powder into the case which does more work for us down-bore, without raising the maximum pressure peak.
The big case, small bore, bottleneck revolver cartridges do bark a lot of fire and hate - but do remain to be rather mild on the business end.