Under the federal law related to firearms (18 U.S.C. Section 922), because a Missouri SIS is not a conviction it will not serve to disqualify you from gun ownership. You are not a convicted felon under Missouri law and therefore are entitled to gun ownership. Now, if there are any little details that need to be considered, you should speak to an attorney as the federal penalties under 922(g) are STIFF! Good luck.
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This is a very sensitive topic and somewhat of a hot button issue these days. The reason is that you are dealing with competing laws: State v. Federal. Most certainly in Missouri you are not considered convicted and have the same rights as everyone else. However, under Federal law the plea of guilty itself could be interpreted to trigger a "conviction" for second amendment purposes. As Mr. Newton stated, you want to be extremely careful as the penalties for felon in possession of a firearm are harsh, however, because you had an SIS I believe you are safe. I have never seen a U.S. Attorney in Missouri attempt to indict someone on felon in possession under the circumstances here. Other states, however, may be different.