High pressure has always been the "weak link" when referring to the 1876. I had the copy of an original test done in the late 1800's, to test the strength of the 1876 action. I don't know where I put it, but it wasn't very scientific as far as pressure gauges and modern testing equipment, but they did put the strength of the action to test.
It was about the same methods as mentioned in the previous post, with nothing more than barrel obstruction, but it definitely was a strength test on the 1876 action. The final analysis was that the action would take more pressure than the reputation it has earned.
I for one can attest to the strength of original actions as I blew up an original 1876 in 45-60. I was loading my ammunition on a Dillon 550 press and had a glitch with the primer feed on the press and while I was messing with it, I ran the ram up an additional time and double charged the case with 5744 smokeless powder.
When I shot the round, not knowing it had been double charged, The gun literally blew up. The barrel was blown off the receiver and the part that was still attached to the receiver was split, causing the receiver to be bulged at the threads. Another round detonated in the magazine tube and blew the forearm into pieces, opening up the magazine tube like a banana. Thankfully, I was shooting in a long range match and was supporting the rifle with my left hand under the action.
Mike Venturino looked at the gun in pieces and we analyzed what happened to put all the pieces of the cause of the blow up together to better understand what went wrong. This gun has been the item of discussion in several magazine articles illustrating what care in needed in reloading and shooting these old guns. Shrapnel is what he has referred to me ever since, the name has stuck for some reason. After all of that...the action held, making me wonder where the reputation comes from about 1876 actions being weak...