As noted previously, I prefer discussions of the M1A/M14 rifle on this forum, but I will mention one other thing that has not gotten as much attention, but I think was the other item that led to FBI's interest in his activities and to his downfall: Clandestine lobbying on behalf of the Turkish gov't, and the implications to his role as National Security Advisor and possibly security clearance concerns.
What others might not realize that is that Flynn's plea deal likely sheilded him from further legal jeopardy regarding some very swampy activity. His two primary associates in his consulting company were charged with illegally lobbying for the Republic of Turkey, but Flynn got off, even though it was op-ed that he wrote and had published the day before the Nov 2016 election that was the main deliverable for the Turkish gov't (the article earned him I think $530k, if I recall correctly).
Michael Flynn associates charged over illegal lobbying for Turkey
...I won't go into great detail here, but I will summarize by saying that anyone who holds or seeks a high-level security clearance (TS/SCI as required for the National Security Advisor) can not
engage in activities on behalf of foreign nations that can create the appearance of a conflict of interest with respect to loyalty to the US and it’s national security decision-making processes. This is especially true
for the National Security Advisor, Director of National Intelligence, Director of the FBI, Director of the CIA and thousands of other highly sensitive roles. Making a public speech or two in foreign countries is one thing for a small honorarium, but having hundreds of thousands of dollars flowing into your 'consulting' company for clandestine lobbying efforts on behalf of a foreign gov't is not tolerated. It will definitely hold up one's security clearance.
General Flynn retired from the military in 2014. He probably never thought he would re-join the federal gov't, and his now defunct company, the Flynn Intel Group had some foreign clients circa 2016, with Turkey being one of them - but it was a purposely hidden relationship with a Dutch company as the intermediary. When he suddenly and likely unexpectedly found himself the appointed National Security Advisor to the new Trump administration, he didn't disclose his lobbying efforts for Turkey. Why? It would be problematic.
I read an article or perhaps in Bob Woodward’s book, Fear,
that Sally Yates (DoJ) privately met with White House counsel Don MaGhan to brief him that there were "some concerns" about Mike Flynn as Trump's national security advisor. It was not clear what these "concerns" where as discussed b/t these two attorneys. Aside from the Russian interactions, I wonder if the clandestine lobbying effort for Turkey was known by the DoJ/FBI. If it was known, and he did not disclose that activity, that likely got him in trouble from a vetting perspective, and my guess is McGahn knew about this, which may have contributed to Trump subsequently firing Flynn, compounded by the lying about the Russian contacts.
So, while is a acceptable for a private citizen with-out a high-level security clearance like Paul Manafort to engage in his unsavory swamp creature activities while extracting millions of consulting dollars from the Ukrainian political class - it is simply not acceptable for the National Security Advisor to a US president to engage in similar activity with respect to foreign investments and foreign lobbying efforts
. Whether or not the 'Turkish issue' contributed to the FBI's interest in interviewing Flynn is unclear, but I get the impression there were "some concerns" from those who might have been somewhat aware of his foreign lobbying activity, if only obliquely at the time. Again, I think its a sad story of warrior who upon retiring from the military immediately entered the swampy world of a DC consultant/lobbyist, and made some errors of judgement... (To my knowledge, John Bolton never made any remotely similar mistakes in judgement over the past few decades...)
Morale of the story? Carefully vet the national security advisor before making a hiring decision.