Complex sentence: Whitworth and Armstrong introduced improved designs in the 1850s and 1860s which were adopted by the British Army, but concerns about armour penetration of the shells and wear rates led the British Government to revert to the RML in 1870.
Split into simpler sentences: Whitworth and Armstrong introduced improved designs in the 1850s and 1860s. These were adopted by the British Army, but concerns about armour penetration of the shells and wear rates led them to revert to the RML in 1870.