One unique feature found on most MP 38 and MP 40 submachine guns was an aluminum, steel, or bakelite resting bar or support under the barrel. This was used to steady the weapon when firing over the side of open-top armored personnel carriers such as the Sd.Kfz. 251 half-track. A handguard, made of a synthetic material derived from bakelite, was located between the magazine housing and the pistol grip. The barrel lacked any form of insulation, which often resulted in burns on the supporting hand if it was incorrectly positioned. 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.