I not an expert, but I think I can shed some light on this.
The strength of a chamber is dependent on the thickness of the walls and the material the walls are made of. Usually this "strength" I'm referring to is resistance to becoming plastic and permanently deforming (i.e. ringing, bulging, exploding).
Rifles vary in construction, depending on the intended use and the SAAMI or NATO operating limits for a particular cartridge. The SAAMI Maximum Average Pressure, which is the average of the peak pressures measured on numerous rifles not the average peak in the barrel, for the 45-70 is 28Kpsi. The Marlin 1895 is designed to safely handle 28Kpsi and may in fact provide an over-load safety cushion of 2X.
It is difficult to exceed the safety cushion with normal loading practiices with the 45-70 in the 1895. However your AR can be destroyed by jamming the bullet into the lands. I don't think the AR design has a 2X saftey cushion.