Yeah, I saw that coming.ummmmm not likely lol I'm thinking about unassing my Vang Wingmaster and putting this in its place.
There were a couple three different variants that I have seen over the years.
The Standard model as shown had a 21" barrel and rifle sights, All the ones was ever around had the Sights removed and were used as standard ribbed or beaded shotguns. Its not impossible to become proficient with them this way but even then I felt it was not the best way to go.
I never seen the synthetic stocked version but plenty of taped up wood. Now days the Corps has much better equipment and has arguably the finest fighting shotgun produced to date, but the MK1's were all wood and steel for along time.
Notice the Sliding stock on the Recon Marines 870 up top. Meeting that guy on a beach on some tropical island in the middle of the night probably didn't end well for a few bad guys.
Another quote from a decent article on them.