You know there's always the chance that the lensman is all right. Given that Sagara is generally an decent human being, even though he's a little lacking in social situations, there's always the chance that the lensman gave him the weapon while in deep cover.

I'm just going to wildly guess that Sagara is still one way or another working for MITHRIL. And the Lensman's in deep cover with the group and he may turn up just fine. Given the fact that Sagara is a good soldier Buck might have figured that Sousuke wouldn't lose it.

Meh maybe I'm just hoping that Captain Testarossa and the rest of the crew of the tuatha de danann show up.