 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 Sdkfz 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 . 
