They are being made in nearly all the gun clubs up and down the country by the members reusing old cartridges and this has been happening for years, so there really is no problem at all.
But on the subject of the plastic gun, the real threat is the problem of detecting anyone carrying one through security check points such as at airports, conferences, political meetings or even sporting events.
I know there are plenty of people making their own ammo, but getting hold of detonators and gunpowder is hopefully beyond the means of the average yob. But that is not the only way to power a projectile.