Jeez, that hard plastic shell is tough all right. I guess it should be but it's a challenge.
This is a device used to cut the much softer outer oval shaped white plastic sheath on Romex home wiring. It's crude, probably this exact device is too crude for the sheaths in question. But the principle is nice. Those sheaths present some problems. They are generally a bit lumpy, with little hills and valleys along the way. A blade depth that would cut nicley through the sheath thickness w/o damaging the wire within would likely barely cut at all in spots. I imagine a more complex device with an adustable depth exacto knife blade mounted. Something that can be sharpened. But somehow I doubt such a thing is on the market and making your own can be a long project.
Sharpening the little nub of a blade on this thing could be dicey - soon you'd have no blade left. OTOH, it's cheap ($2.67), one could practice modifications on a few w/o much expense. Such as making the opening on the one end bigger, maybe even one sided so you could slip the thing over the sheath in odd spots.
https://tinyurl.com/y4rxr4rb