I probably should have added the commentary earlier. The first video shows footage of the Salvo12 silencer from SilencerCo. http://www.silencerco.com/salvo Youtube of first video.
What's the mistake? If it's the position of the suppressor, the RMB-93 does have the barrel below the magazine tube.
Edit: Speaking of 155mm suppressors, the Bundeswehr's WTD 91 in Meppen, Lower Saxony has (or, I believe, had) one built by Rheinmetall, MAN, and IfL. Rheinmetall also has a US patent on "[a] silencer arrangement for large-caliber guns".
Rumor? No, there are Shotgun Silencers out there. Just you have to specially request them. They aren't typically used. Not saying they aren't being used. They are. Just that you don't hear about it. As people are silent about it. You know government secret stuff. Hush hush kinda of BS.
Plus it is illegal to use something like that hunting....it isn't sporting.