By your definition a belt fed machine gun with a single shot setting is an assault rifle.

Only if it’s in an intermediate caliber and light enough to shoulder fire.

Which, admittedly, there are a few of those out there … but only a few. And, well, arguably such a gun could be an ‘assault rifle’ if you chose to use it as one. Being belt-fed rather than magazine-fed doesn’t really fundamentally change the nature of the thing, and at that point, the feed mechanism is the only functional difference between that and what you’d typically call an assault rifle.