The barrel spring was intended to correct a problem with the .380s whereby the empties sometimes struck the ejector (which was a part in common with the .32s) too far forward causing a stovepipe. The spring pushed the barrel to the right just as the slide approached the end of its travel curing the condition. This was done by the Shanghai Police armourers, who sent a report to Colt about the problem, and the final Shanghai Police order has this spring put in at the factory. Nobody else seems to have noticed this unreliability, probably because few users ever shot their pistols that much. In 1945 the U.S. Navy discovered (or rediscovered) the same problem, which Colt had decided to ignore unless the customer was big enough to be worth appeasing, and sent all as-yet unissued .380s back to Colt for correction. Colt’s engineers came up with a better solution than the Shanghai one, designing a gauge to measure whether the empties would hit the ejector at the right point and then grinding it down until it worked properly. Magazines were also altered as some of the lips were too narrow. It is not known whether the special SMP magazines were an attempt to address this problem.
Guns and magazines which received these modifications were stamped with a M (prefix to the serial in the case of the gun).
About half the US military .380s received this treatment.