Hi, As far as I know the standard drone 2 barrel does not take inserts. If you want inserts/sizing you'll need a kit like the shift 2, a freak, or sized barrels (all autococker threads).
This is from the MacDev website:
A serious barrel
The Shift 2 barrel system used on high end MacDev markers has been modified for release with the Drone 2. The insert system has been removed to create a Shift 2 lite version. The same great performance, with a solid 2 piece design. All parts are compatible with the big brother full Shift 2 so you can easily upgrade to a full insert system later.
I personally use a freak kit on the Drone 2 and have great results.