You mean this from early Winchester records?
"The strength of the Model 1876 rifle and the .45-75 W.C.F. cartridge was tested by Winchester in the late 1870s. The factory conducted tests on the strength and reliability of the action to answer concerns by customers. These tests will astound collectors and shooters who have stated the Model 1876's toggle link action is "weak." In response to a letter sent to the company by Charles Hallock, Esquire, of Forest & Stream magazine, Oliver Winchester responded by telling about the tests the factory accomplished on the 1876 rifle. He indicated that engineers first started the tests by removing one of the toggle links and fired 20 rounds (this was with .45-75 W.C.F. cartridge with 350 grain bullet) with no effect. They restored the missing link then went through 6 more trials starting with a charge of 105 grains of black powder, behind a 700 grain bullet! The comment "worked well" is noted. They then increased the charge of powder to 165 grains behind 3 bullets (1,150 grains) and that "worked well." From there, they increased the powder charge to 203 grains and added more bullets until they reached 1,750 grains of lead (five 350 grain bullets). This also "worked well." Finally, they added one more bullet, bringing the total weight to 2,100 grains, and things began to happen. The comment was, "Breech pin slightly bent. Arm working stiff." The seventh and final test was again 203 grains of powder but this time six Martini bullets weighing 480 grains each (2,880 grains) were used. "The charge bent the breech pin, blew out the side plates, split the frame and otherwise disabled the arm," was the comment. Oliver Winchester noted that in this seventh trial, the shell had burst into fragments and the escape of gas at the breech did the damage."
Being active in the gun industry at many levels the last 30 years I have seen so much ignorance represented as "fact" I make a point of checking the well know "facts" out myself. Beginning to think the "weak" '76 was a historical "fact" based in fiction.
Knowing that a 1873 is now chambered for a 44 magnum and the resulting 36,000 PSI loads I think we are safe loading the 45/75 with smokeless. I agree with Silver Rings the modern 1876 is stronger than most think. And the fact as Silver springs has mentioned Uberti lists the gun (by an article on their web site) as capable of 29,000 PSI. Easy to stay under that with 300/350 gr smokelss loads I think.