Complex sentence: Britain had successful tested a new HAA gun, 3.6-inch, in 1918, in 1928 3.7-inch became the preferred solution, but it took 6 years to gain funding.
Split into simpler sentences: Britain had successful tested a new HAA gun, 3.6-inch, in 1918. In 1928 3.7-inch became the preferred solution, but it took 6 years to gain funding.