Kalashnikov Media has published an article telling the story of a very interesting device designed to allow feeding linked ammunition into standard magazine-fed RPK-74 light machine guns. This device was called SPU (СПУ – Съемное Подающее Устройство, stands for Detachable Feeding Device) and it was developed in the late ’70s right after the adoption of 5.45x39mm cartridge and AK-74/RPK-74 weapon systems.
Shortly after the adoption of RPK-74, Soviet military officials required to design a new 5.45x39mm LMG with an increased downrange effectiveness in terms of the volume of fire. To develop such weapons they launched a project called “Poplin”. There were several weapons designed within this project and the main solution was to make an LMG that could be both belt-fed and magazine-fed (like the M249 SAW). Along with the LMGs designed during this project, an IzhMash designer named Vladimir Kamolov came up with the SPU feeding converter. This ingenious little device allowed to feed the RPK-74 with linked ammunition without any changes to the LMG’s mechanism whatsoever.
This device is attached to the RPK-74 just like a standard magazine. On its bottom portion, the SPU has a provision to mount an ammunition box containing 100 or 200 rounds of 5.45x39mm ammunition loaded into a non-disintegrating belt. The belt is fed from the right side of the converter. Inside the device, the linked ammunition is squeezed upwards from the belt links and gets presented for feeding in a pattern identical to the magazine feeding. The mechanism of SPU feeding device is powered by an external lever that is attached to the charging handle of the weapon and powers the mechanism using the reciprocating motion of the bolt carrier group. The empty belt comes out from the left side of the converter.
Although this system did increase the capabilities of RPK-74, it also had shortcomings. Particularly, it made the gun much heavier and bulkier and also increased the chances of a cook-off because the RPK-74 fires from a closed bolt. I think the excessively large volume of fire from an RPK-74 is also not desirable considering that this LMG has no barrel change capability. Due to the mentioned drawbacks and other issues associated with adopting and fielding this device, the Soviet military eventually decided to abandon this idea. There are only several experimental samples of SPU devices surviving to our day. The sample shown in the images is kept in the R&D department of Kalashnikov Concern.
Images from www.kalashnikov.media