My point is the legal argument. From the document above, I think a key feature that will help define an airsoft gun is that it is not designed to handle these dangerous ammunitions. If this reccomendation is put to the commons and the Lords, it only takes a few loud people who feel strongly about RIFs (like the greens and labour) to require an airsoft gun to not be readily convertible to handle any missile other than a sphere.
It's not my opinion, its just how I believe the document would be interpreted.
And yes, you would need to replace the barrel, hop chamber and magazines, but most of us on here have done this already, only with aftermarket parts rather than custom parts.
Thinking now, I don't think that an airsoft gun could be considered readily convertible this way...