Look at the internal cuts prior to making the external ones - especially be mindful about preserving certain segments of the top and bottom of the internal shell.
External cuts overview:

Internals prior to cutting:

Making the initial internal cuts:

After internal cuts:

Installation after completing cutouts:
https://www.reddit.com/r/Nerf/comments/1cb7vos/printed_hammershot_rearload_guide_installation/
The author remixed this model.
While technically a remix, nearly every face has been altered.