The only similarity is the shape of the case. I've owned and shot both. The 120 is a black powder cartridge for use in rifles of the era, Sharps in particular. The standard bullet is cast lead in the 500-550 gain range with velocities of about 1200 fps. It is a very effective long range killer of large bovines. The Brit 3 1/4 is a nitro cartridge for modern era rifles. The bullet is usually in the 480 grain range at 2150 fps. It's an effective killer of about anything, usually at short range due to the rifles it is used in. Basically it is the higher velocity of the Brit that gives it the edge in foot pounds. Given a rifle of sufficient strength you can load the 120 to match the brit, however the 120 is usually offered in black powder replica rifles. It's just good common sense and a desire for self preservation to load the cartridge to take into account the mechanical weaknesses of the rifles rather than "put the juice to it" and blow ones self to hell.