SIG Stgw 57 « Forgotten Weapons
IIRC, one of our fellow HKPro members had a HK416 10.4" mounted on his motorbike. It looked really awesome!
With regards to the neighbors in Wisconsin, the local ordinances allows one to shoot on one's propert as long as there is a berm.
I found that out when we were shooting full auto bursts the first time the RDIAS permits were approved a few years ago, and the police officer told us we could do so with a berm; different story that I'll post about when I have time. !