"Mike puts the small weapon he took into [or inside] his jacket."
Mike puts the small weapon he took into [or inside] his jacket.
Also saying "Mike takes the small weapon and inserts it into his jacket" is not considered particularly complex, and both forms are acceptable. One note is that using two distinct clauses combined with the conjunction "and" can be more clear, as consider the following:
Mike inserts the small weapon he took into his jacket.
Did Mike take a weapon and put it into his jacket, or did he "insert" the weapon that was in his jacket into something/someone else? We cannot tell from the sentence alone which is intended, and thus the use of two distinct clauses: the first clause notes the taking of a weapon, and the second clause notes what is done with the weapon that was taken.
