One autopsy-—there have been three, to date-—indicated Brown had an injury at his thumb joint-—which might indicate a struggle w/ a hard object, like a gun.
If he did struggle w/ the officer’s gun-—he would have left his DNA there.
Might not even need DNA. One good fingerprint would prove Brown's hand was on that gun. Dorain Johnson has already admitted that he lied about what happened in the car, and that it was Brown that attacked Wilson, not the other way around.
The number of autopsies is questionable. The first autopsy was performed by the St Louis medical examiner. The supposed second autopsy was done by a fake non-doctor named Shawn Parcells. Dr Badan did not participate in that autopsy and arrived in town two days after it was completed and only commented on the one done by the fake non-doctor Parcells. Attorney General Holder was going to have a third autopsy done but to my knowledge that has not been completed yet.