Rather than a new "weapon" weapon, I propose a different usage for this design. Oftentimes I am using the Rescue Ranger to keep alive a sentry in a very forward/difficult to reach position, and it usually works well, for a while. Eventually, the sentry will run out of ammo, and although the resuce ranger works very nicely for keeping it alive, it unfortunately does not reload the sentry. I have always thought that a secondary that can reload a sentry would make for a perfect partner with the rescue ranger, and this gun does look the part. I propose the following:

Each shot reloads 10% sentry ammunition, it has 20 rounds of ammunition carried, with 4 shots per round. firing speed should be similar to the rescue ranger, slower than default pistol. deals 10 base damage per bullet, with similar scaling to the pistol.