Complex sentence: Due to patent restrictions on the 1905 design, Winchester was unable to copy the Browning design they had rejected earlier, the only autoloading shotgun design at the time, so Winchester had to adapt the design for their own production without infringing on Browning's patents.
Split into simpler sentences: Due to patent restrictions on the 1905 design, Winchester was unable to copy Browning's design that they had rejected earlier. Since Browning's design was the only autoloading shotgun design at the time, Winchester had to adapt the design for their own production without infringing on Browning's patents.