The toy guns are made of soft metal that isn't even capable of handling the force of a firing live ammo. Replacing all the parts would be more expensive than buying a real machine gun rather than converting a toy one even if it was possible. Even if the toy gun was successfully converted, it would blow up when fired. Seeing as it is impossible to play transformer and make these toys into real functioning guns, where's the justification?  

(Read More about how far-fetched it is to transform toys into guns)