I'm the gunsmith\mug who is considering this idea
Basically back in the UK before the Brocock ban I was making one-off rifles for people who didn't want the hassle of an FAC, but wanted something more functional than a deac.
For example I built an L96A1 replica which used a real AI stock and chambered the old brass Saxby Palmer cartridges used in their Herald and Enfield rifles.
Another project was a Nesika Bay style single shot rifle, all stainless, with a heavy barrel.
There were only two people in the UK doing this sort of thing to my knowledge, and the other guy was and still is a good mate of mine. He built an air cartridge AK47 which was commercially marketed, but it all went wrong because the company selling it took orders knowing full well he couldn't build them that quickly.
It wasn't a big market because the guns were very expensive. Due to legal reasons you can't convert a deac into an air gun in the UK, so I machined bolts, receivers, etc from scratch. Virtually the only things I didn't make were the barrels ( although I still had to chamber them and machine the blank ) and trigger units ( I used Remington 700 triggers ) Put it this way, the L96A1 was well over four figures.
Of course the Brocock ban buggered everything up and that was the end of that.
Now in Spain I can't see a legal impediment to converting a deac into an air rifle or pistol. Immediately that lowers the price of the finished article because I don't have to make it all from scratch. Secondly the Spanish idea of deactivating a weapon is usually a complete joke, so I've got most of the parts there to work with.
The shape of the TAC cartridge is the limiting factor, it's very blunt nosed and going to have to feed out of a magazine it was never designed for. This would limit the actions and calibres suitable. I'm actually mainly thinking of the Micro Tacs in .380 or 9mm pistols. The full size Tacs would go well in .38 special revolver actions.
Maybe a nice Kar98 sniper using the Saxby Palmer Tacs if I can get any.
At the end of the day it's an expensive toy, but a very cool toy which doesn't have stupid sanctions on it's ownership.
I don't know how it will all unfold though because our house sale has fallen through and I'm going back to the UK in a week.