I was bale to handle and fire a TK M4A1 myself. That was a really great day and I won't forget that.
As you may know I am a nitpicker, and I have to say that some details on the TK M4A1 are not fully correct, especially the markings. And its biggest drawback is that it is made from plastic.
BUT handling and firing this model makes you forget about its drawbacks or trade-offs. It fires like hell, in semi, full auto and in bursts. I was able to fire 70 rounds, and only 2 failed. I was told by the owner that it was the first time the gun failed at all. We noticed the carts had "mushroomed" a little, getting wider in the middle. I have noticed that to happen to MGC .223 carts as well!
The best modelgun I have fired so far, and well worth its money I'd say.