Just received my latest issue of SARand in an article on folding subguns there are some very interesting concepts done by the French in the 1950s and 1960s. The first was made by Hotchkiss and called the Universal. To make it even more compact than just folding, its barrel can be 'telescoped' into its receiver. when folded and telescoped, the Universal is only 6.5 inches longer than its barrel.
The most unusual was the M.G.D., more commonly known as the P.M.9. The breech is a spring-loaded, oscillating flywheel. It rotates on a center axis. A short bolt, inside of the flywheel, does reciporcate far enough to extract the fired round. As it retracts, the bolt imparts a spin to the flywheel and the extracted cartridge is ejected from the top of the P.M. 9's receiver.The P.M. 9's innovative (and expensive to manufacture) breech allows its length to be less than 14.5 inches when folded. With an easy modification, the P.M. 9 could be even shorter. When folded, its barrel extends past the folded magazine. If the P.M. 9's barrel were shortened to the length of its folded magazine, its overal folded length would be 12.5 inches.
Above from SAR Vol.3 No.7 ppg 36-7