Although the MP 40 was generally reliable, a major weakness was its 32-round magazine. Unlike the double-column, staggered-feed magazine insert found on the Thompson M1921/1928 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.