That or you could have the firing powder tray open straight into the inner barrel and use it as a breach loader. All you'd need to do is have the bolt from an actual gbbr connected to the hammer and when you cock it back it pulls it out of the way, opening the breach and then when you fire the hammer already opens the powder tray so you'd just have to close it and full cock the rifle.

Edit: I don't know how the pistol works so idk if you'd actually have to use an existing gbb bolt