While a neat idea for training, perhaps some live fire exercises, it defeats a lot of the versatility of an air gun.
One of the nice things about an air gun is that not only is it safe, but the law most places has fewer restrictions on air guns in most states.
This allows you to take and use an airgun in places where you cannot take or use a firearm.
By mixing the two you are playing on the line.
When you shoot it clearly you are shooting a pellet with air, an airgun in some state laws.
But you are also using and possessing a firearm.
This is going to confuse many when it comes to the law.
Are you discharging a firearm when you shoot it? Probably not most places, but perhaps in some, and how do the local cops interpret it when they come to investigate the person reported to be shooting or with a gun (pellet, bb and even airsoft shooters will have this occur)?
Is possession of this a crime in places where one cannot possess a firearm, but pellet guns are legal to possess?
It is probably not an assembled firearm per the law, but it certainly complicates things.
If you put a barrel under 16" on it and a stock, is it an NFA item? Common sense tells us no, but this is an area one will need new ATF opinions.
What if you modify it to be full auto, does it become a machinegun?
Where is the toy lowers when you need them?
Oh right, the ATF declared them to be firearms because they could mate to actual uppers and prohibited their import.
(Not that it would have done any good since they cost several times what an actual lower does.)
So I like the concept but it does make using, possessing, and transporting, and airgun more complicated than just an airgun normally is.
WHAT? 
