Complex sentence: Sturm, Ruger introduced a number of new lever-action designs in the 1990s, which is unusual since most lever action designs date back to before World War II, when reliable semi-automatic rifles became widely available.
Split into simpler sentences: Sturm, Ruger introduced a number of new lever-action designs in the 1990s. That is unusual because most lever action designs date back to before World War II, when reliable semi-automatic rifles became widely available.