Complex sentence: The MP 40 also had a forward-folding metal stock, the first for a submachine gun, resulting in a shorter overall weapon when folded; however, this stock design was at times insufficiently durable for hard combat use.
Split into simpler sentences: The MP 40 also had a forward-folding metal stock, the first for a submachine gun, resulting in a shorter overall weapon when folded. However, this stock design was at times insufficiently durable for hard combat use.