I tried an eBay Search for 'mattell thunder burp' and got hits for the Thompson w/drum clone
(of which I had one also) but not the M3 clone. My [Thompson] ThunderBurp had the compensator broke off and no stock (if they even had one). I have a pic of it somewhere here...I'll have to look...
But the M3 toy was the cool one. Actual tin like the real one. And it shot caps! (The [Thompson] ThunderBurp didn't shoot caps; it had a sound chamber and a wind-up mechanism that made shooting noises).
Edit: Found the pic! Lookit that cute little guy...8 yrs old...With his cap in one hand and toy in the other...I don't recall ever owning that kind of cap...No idea where it came from...