My argument has always been this: Yes, they are the same gun, but the different designations are helpful in determining how that gun is configured. To use your k98k and G98 example, yes, they're the same gun, but from the names we know the k98 is the shorter model, just like the 91/30 and M44 are both Mosin-Nagant rifles, but we know the 44 is a shorter, carbine model. Likewise, the colt commando family has features that distinguish it from the M16.
Is the AK-200 the same gun as an AK-47? Yes, essentially, it is, but it has its own features and changes to the design over time that differentiate it from the original.
Also please note that from a semantic perspective, the designations also refer to the purpose of each gun- different purposes I believe are an important enough factor to consider them different guns. Carbines are designed for lower ranges than full sized rifles, so to say an M16 and an M4 are the same sort of cheapens that distinction doesn't it? And what about the Colt SMG? It's the same as an M16? Well it fulfills an entirely different role, so how could that be?
That's why I use the different designations to refer to the weapons. Yes, they're all AR-pattern rifles, but each designation has encoded into it some very specific descriptions of how the rifles are configured. Again I'd like to reiterate that this doesn't apply to civilian rifles, as these rifles have the potential to be a hodgepodge of all sorts of different parts that defy classification. Even so, if you bought a premade AR of a certain model from a company, the model name of that gun would have encoded into it the same information that a military designation does.
So when I say that the Assault Carbine is a CAR-15/Colt Commando, what I'm really saying is that the features of the ingame weapon that I can identify most closely match the features of the real life Colt Commando, insofar as the military designation implies that certain features will be present.
Also, the marksman carbine will probaly fire 5.56, but remember that the assault carbine fires 5mm, so who knows what the MC will fire.
The 5mm is supposed to be a weaker round than 5.56, which is why it was used in the minigun, IIRC. So if I was going to choose a round for my marksman carbine, I'd choose 5.56.