Loose Rounds posted this on their Facebook Page. It is a Remington 7188 used in Vietnam. According to Gun Wikia it is a modified Remington 1100. I imagine the recoil would be somewhat brutal but could be mitigated with backboring and porting like what Vang Comp does to their 870s. 480 rpm is a rather slow rate of fire and might not be too bad if set up properly.
Here is a photo of the selector switch.