These rifles were made when metallurgy and understanding of pressures were in its infancy. They did a lot of things which would be considered very sketchy today at this time period. Generally, a lot of things operated at the bare minimum margins of safety. Many nations converted their older black powder designs to smokeless (or attempted to). Many had issues due to the fact some of the rifles were not up to snuff for the higher pressures smokeless powder generated. Even some dedicated smokeless powder rifles had issues in this time period (best example being the Gewehr 88).
Trapdoor Springfield conversions to .30-40 Krag (not a good idea). French Gras conversions (I have one personally in 8mm Lebel, fun rifle to shoot but I stick to the low end of the pressure scale due to the limitations of the design). The Vetterli 70/87/15 rifles, which wasn't even that good a conversion as many had feeding issues excluding the obvious pressure issues. Martini-Henrys to .303 (not actually a bad conversion). etc.
I personally would approach shooting one with extreme caution myself, no point in getting hurt or destroying a 150 year old rifle just because you want to push it to the limits.