Complex sentence: In the USA the enormous number of war surplus muzzle-loaders produced the Allin conversion Springfield in 1866, with a trap-door loading mechanism of the British gun, the firing mechanism was very similar to the Prussian gun.
Split into simpler sentences: In the USA, the enormous number of war surplus muzzle-loaders produced the Allin conversion Springfield in 1866. With the trap-door loading mechanism of the British gun, the Springfield's firing mechanism was very similar to the Prussian gun.