"Modern loads" generally means factory, readily available at the hardware store shells. Basically, you need to stick with modern guns to shoot those. There are a number of reasons including proof pressures, wear and tear on the gun and on yourself, as well as chamber size. As the owner of multiple hammer guns, I can tell you that owning a modern gun is much cheaper in the long run. I love my SIACE for its reliability and the fact that it shoots just fine with whatever I put through it. No worries and it's not too light weight.
Each of my 1870s-1890s hammer guns requires a relationship with my gunsmith and special formulae for care and feeding. Even my Husqvarna is happiest with RST and other low pressure loads.
