I wouldn't go by Hollywood's gangster movies any more than their westerns for an accurate portrayal of the era.
The Tommygun was used by both the bad guys and the Feds, and some local police as well. Machine Gun Kelly got a name and a rep but apparently used other guns more than the Tommygun. 2 Tommyguns are reported used in the St Valentine's Day Massacre.
I think it was the high profile nature of the gun's use, rather than the frequency that made it so iconic.
I have no idea if the FBI collected statistics on them in those days, but they probably did. No idea how you'd go about getting them, though, sorry.
While you're looking at Tommyguns look at other full auto weapons used by famous "motor bandits" "whippets" made from sawed off (stolen BARs) were used by Bonnie and Clyde and I think a few others.
I remember seeing a 1911 pistol (.38 super) that is in (the FBI?) museum, converted to full auto and with a forward pistol grip. taken from "the Dillinger Gang".
Absolutely they were used, but I think Hollywood used more Tommyguns than the real bad guys did.
Moving to the Full Auto forum...