That is a really great modelgun, an exact copy of the real one, featuring the three-round-burst mechanism. Sadly it is a dummy only model so you can only enjoy this feature a little when cycling it manually.
It was made by HobbyFix in 2001 as the last model of their M4A1 and M16A4 series. Numbers are very limited and it was and is very expensive.
I guess this has lost some of its appeal as there are very realistic AirSoft M16A4 models available now, but back then, its realism was almost unique.
Thank you, Cerwyn, for putting the pictures up here.