Has anyone seen the slide fire stocks in person just got magazine with them in it for 350 bucks looked it up on you tube. And they look like they work great but is it legal to own in de looks like full auto to me
Its not full auto. They use the gun's own recoil to chamber the next round and you control rate of fire with your forearm pressure.
They come with a BATFE certificate stating they are perfectly legal. Also check out the DEFENDAR-15, a competing product which is slightly more expensive but has some advantages- all metal construction, you buy one stock and its interchangeable between other AR's or AK's, you just have to buy separate buffer assemblies.
Hickok45 on Youtube explains it well.
Probably the biggest downside to these things are the current ammo prices and availability.