Frank, this is a good concept. It allows you to create weapons that behave differently without the need to buy a whole new weapon. This is very economic and would be fun to toy with in a game.
Yes. Imagine a weapon that evolved differently dependent on the components that assembled it. It would be like Digimon or DigiGun should I say - hehe. The only problem is that the interchangable parts concept means large heavy weapons
Maybe...Maybe not. If every aspect of the weapon can be upgraded it will not matter. For example, in a Sci-Fi setting, lets say you upgrade to the "Utopia" Chamber that features a Triple Chamber design with 3 barrel/Magazine/FireUnit slots. With the "Utopia" your "Devatron" weapon can be assembled into a Machine Gun/Bazooka Launcher/ Sniper Beam Weapon.
Of course, the "Utopia" is compact, and so are the "Grittle" Machine Gun Fire Unit, the "Z-Range" Sniper Barrel/Fire Unit/Magazine Assembly, the "Hercules Spitter" Magazine and "Gorge" Heavy Barrel that construct your "Devatron".