On May 9th North Korean claimed, via a video clip showing a SLBM (submarine launched ballistic missile) test happening behind leader Kim Jong Un, that it now had a submarine that could launch a SLBM. Up until then it was believed that North Korea was building a 1,200 ton sub that might be able to fire an older type SLBM. As for the SLBM, North Korea was believed to have one or more 1960s vintage Soviet SLBMs but had not yet successfully copied the technology because there were not yet any tests of such a missile. Thus a careful examination of the May 9 video showed it was faked. The most likely scam was to rig an underwater launch tube close to the surface, show it near a surfaced submarine and then launch a missile that would not go very far, just far enough for a photo to be taken.
There were later reports the faked SLBM claims angered many North Korean personnel actually working on sea launched ballistic missiles. The angry North Koreans believed that false reports like this simply bring unwanted attention to their effort and the increased possibility of a pre-emptive attack to prevent such a North Korean weapon from becoming operational. Actually, a lot of people escaping North Korea in the last few years have reported dissatisfaction (among weapons developers) with the growing use (by Kim Jong Un) of false claims about new weapons developments.
What was not faked was the fact that North Korea obtained all or parts of a Russian R-27 SLBM in the 1990s. The R-27 is 1960s vintage tech that was replaced in the 1970s by more modern designs. But many of the unused R-27s produced were recycled for scientific research until 1990. Some 500 R-27s had been launched with an 87 percent success rate. It was believed that all or much of at least one missile was illegally sold as “scrap” to North Korea in the 1990s. This was deduced from the fact that after the Soviet Union collapsed in 1991 North Korea bought a lot of discarded Russian weapons for scrap (none of which was supposed to be operational stuff) and it was later discovered that some of the scrap was remilitarized by the North Koreans. Thus it was no surprise that the new North Korean Musudan ballistic missile looked a lot like the R-27. There are pictures of the Musudan mounted on a large truck (that serves as transport and launcher) not a submarine. So far there has been no evidence of a Musudan test. Typically a SLBM is tested from land facilities before it is tested from a submarine.
North Korea also received ten decommissioned Russian Golf class SSBs (ballistic missile diesel-electric submarines) in 1993, to be turned into scrap. The Golf class boats used the 16 ton R-21 SLBM, which is thinner and longer than the R-27 that replaced it (in the first Russian nuclear powered SSBNs) in the 1960s and 1970s. Foreign intelligence agencies have been watching North Korea carefully for signs that North Korea was working on an SSB but the only possibility found was one new submarine under construction. It did not look like a copy of the Golf class boats but did have a sail that might have held an R-27/Musudan type SLBM. In any event this boat is still under construction. The scrapped Golf boats would have enabled the North Koreans to examine the first generation SLBM launch equipment, in which SSBs fired missiles from a elongated sail structure that contained three SLBMs.
It was a military parade in North Korea that featured the first public appearance of the long rumored RSM-25 (or Musudan) missile. This was apparently a variant of the 14 ton Russian R-27 SLBM. The North Korean Musudan appears to be heavier (20 tons) which indicates a range of 3,000 kilometers or more. But there have been no tests, so it's uncertain if the North Koreans have been able to make the Russian design work for them. The R-27 was Russia's first true SLBM that could be launched underwater. Sixteen of them were carried in Yankee class SSBNs. The 14 ton R-27 had a range of 2,800 kilometers and used storable liquid fuel. This means it can be ready for launch in less than half an hour. It was noted that the May 9th missile did not have the distinctive exhaust indicating storable liquid fuel.