 Although the MP 40 was generally reliable , a major weakness was its 32 @-@ round magazine . Unlike the double @-@ column , dual @-@ feed magazine insert found on the Thompson <unk> @-@ 28 variants , the MP 40 used a double @-@ column , single @-@ feed insert . The single @-@ feed insert resulted in increased friction against the remaining cartridges moving upwards towards the feed lips , occasionally resulting in feed failures ; this problem was exacerbated by the presence of dirt or other debris . Another problem was that the magazine was also sometimes misused as a handhold . This could cause the weapon to malfunction when hand pressure on the magazine body caused the magazine lips to move out of the line of feed , since the magazine well did not keep the magazine firmly locked . German soldiers were trained to grasp either the handhold on the underside of the weapon or the magazine housing with the supporting hand to avoid feed malfunctions . 
