Ultimately, roughly half of the KP/-31s in Finnish service were of the SJR version. The KP/-31 was unusual in that it had a replaceable barrel secured with four lugs rather than threads. Soldiers were issued at least one spare barrel. In close combat the weapon would overheat in sustained automatic fire, requiring the barrel to be changed. This was effected using a mitten or piece of thick cloth to secure, rotate, and remove the barrel jacket. Then the user would use the point of a "puukko" knife or a cleaning rod to loosen and remove the hot barrel. It was easy to then slide in and lock a fresh new barrel, secure the jacket, and resume firing. The Finnish military initially issued it with the 20-round staggered-column magazine and early 40-round "sissilipas" ("ranger magazine") drum. The magazine's capacity was found to be too small for sustained fire. Up to 5 additional rounds could be loaded into it, but it was found that this damaged or over-compressed the springs and caused it to fail. The drum was found to be hard to reload in action because the backplate had to be taken off and the bullets loaded inside. The drum also had an open "shot count window" in the bottom of the drum; the numbers 1 to 40 were stamped on the feed plate and lined up with the window to indicate the shots remaining. However, it naturally let in dirt and debris that led to jams. Aimo Lahti experimented with a 60-round Thompson-type spring-loaded clockwork drum to replace it, but it was never adopted. They were later replaced during the Continuation War with the Swedish-designed 50-round "coffin" magazine and improved 71-round drum. The "coffin" magazine carried more ammunition than the box magazine and was lighter than the 40-round drum. However, it was complicated and prone to jamming or failure if it was damaged, reassembled incorrectly, or the springs gave out. It would also fail to fire or feed unless it was properly seated in the magazine well. The "coffin" magazine was withdrawn from service in 1943 in favor of the 71-round drum. The larger drum was a design by the weapon's less-well-known co-designer, Lieutenant Y. Koskinen, that was an improvement of the 40-round drum. It had a removable front-plate that was quicker to reassemble and was cocked by rotating the mainspring for up to four detents, allowing the drum to be partially reloaded.