Mossberg makes one pretty similar to what you mention but then so do lots of other folks. With no more info than you provide it's gonna be pretty much impossible to identify the specific gun you saw. Did it look like either of these?
I've been thinking seriously about picking up one of these for myself.
It's the 590 Special Purpose 9 shot 12 ga. 20" special heavy-walled barrel with matte blue finish, black synthetic stock and ghost ring sights.
I saw one just like it but with the heat shield installed for sale at a shop but the guy I was dealing with kinda pissed me off so I won't be doing business there anymore. Here is one with the heat shield but set up differently.