A little late to this one, but I FINALLY got to try some of my case-full BP rounds for 45 Colt. HOLY COW, they DO thump I was fearing for the gun! However, I am pretty darn sure it was built for this. This is the second range trip, trying to chrono the loads, but was too close and the smoke interfered with the readings. However, you can see the recoil.
That load was "loaded to 1/8 inch of the top of the case with OE 3F powder and 200gr bullet with SPG Tropical lube". Very accurate load, but a real thumper that creates it's own smoke screen, too.
i WAS going to get an 1875...but I need a matching revolver for CAS, (just joined SASS last week), so it looks like another Birds Head 1873 Uberti in .357.