There are also semi .50's, and these are vastly different. The sideplates are thicker so the FA parts can't be inserted into the receiver, the internals are thinner, and the trigger mechanism is entirely different.
He's exactly right. Though they're cosmetically similar & share some parts (barrels, tripods, etc), I'd think of them as two separate guns due to BATF regulations. The same way the semi versions of open-bolt machine guns are all closed-bolt.
To add in my totally unasked for opinion, having shot quite a few MGs on the government dime and having played with a few semi-auto belt-feds on my own dime, I have no idea why anyone would want one. They're expensive to feed, of dubious utility & practical accuracy and lack the coolness of full-auto. But if they float your boat and you have the $$$......