Sometimes we make laws that very clearly define what is criminal activity. For example, knowingly transferring a firearm to a felon is a crime even if you only do it once.
Sometimes we make laws that are a little fuzzy. For example if you "engage in the business of manufacturing ammunition without a license".
The Congress could have simply made it illegal to sell ANY ammunition without a license but they chose to write that law with a little wiggle room. Probably because the goal of the law was to encourage people actually in the business of manufacturing ammunition to get a license.
They didn't want to criminalize every little transfer of a piece of lead.
This is similar to laws that don't require every charity bake sale to submit to health department regulations, zoning regulations and countless other laws that are really aimed at businesses.
It DOES matter how the law is written. If the government wanted to make every transfer of every bullet subject to licensing requirements - they could have written the law to reflect that. They chose not to make it that stringent.
It is wise to not test the limits of the definition of "engage in the business of manufacturing ammunition".
As for Michael Flynn pleading guilty to lying to a federal agent - He wasn't "forced to plead guilty" he CHOOSE to plead guilty and by doing so, he reduced his exposure to prison. Had he gone to trial, there's no guarantee a jury would have found him guilty. However, if he went to trial and was found guilty, the likelihood of at least some prison time was greater. So he must have felt the government had some credible evidence and he opted to take the safer path.
I don't trust the government but I do believe that it is extraordinarily difficult for the government to obtain a conviction. The feds have a high conviction rate because they ONLY take cases they think they can win. Unlike state courts, the feds don't have to try every rape, robbery and murder. They get to pick and choose what they prosecute and they cherry pick only the strongest cases.