Even if nobody could have obtained the guns from the study at that time, if Krauss or Kumasawa is the culprit, since they regularly visit Kinzo, they could have secretly taken one of the guns at some times prior to the murder. Alternatively, Genji is only an accomplice, and someone asked him to get the gun for them - in that case, the culprit could be anyone without an alibi. It's also possible that the culprit had a gun prepared from elsewhere, and did not need one of Kinzo's Winchesters.
But, am I missing something, or is the crime basically solved now? Wasn't our purpose simply to prove that the crime is possible for humans, which has been done? Or do we have to show who the culprit is as well?
Well, at this point, it's either Krauss, Genji, Eva, or Hideyoshi.