You're not the only one to have had this idea (nothing to do with me, despite DCL_Dave's attemot to smear me with it.
Vintage Airguns Gallery
..Above link posted with permission from Gareth W-B
In British slang an anorak is a person who has a very strong interest in niche subjects.
Good deals with these members
Worst I have seen recently, from a police amnesty hand-in, was a Mk5 Airsporter that had the barrel sawn off at the end of the receiver tube. As is the way of these things, it was slated to be cut up and melted down at taxpayers' cost, rather than sold to the boller-suited Ant and Dec at Protek (you guys are great, sorry) to be broken up for spares.
The scariest (including in user terms) illegal conversion that I have seen in a while is an airsoft blowback pistol very very badly converted to fire live rounds. Or a live round. Maybe. It appeared that the conversion involved tools from Poundland, wielded by an idiot. Zamak meets 9mmP 2Z, becomes hand grenade.
Its an odd choice for a conversion. A functioning 'zip-gun' can made up from tube etc, rather than as you say making a non-functioning self-bursting thing. Perhaps its all about the looks.
An 'official' zip-gun is the USA's FP-45 Liberator pistol, but I guess it doesn't look as scarey as an airsoft....
https://en.wikipedia.org/wiki/FP-45_Liberator
i once fitted a ox spring to a gamo statical that's up there with a gas stud crap