
590R_InUse_Adjusting_2911x1943
(PC - Mossberg)
This past October, the Mossberg crew hosted several gunwriters at Gunsite. More than a few of the group were or had been working street cops. We were all there to examine the newest additions to Mossberg’s defensive shotgun line.
Rich Kirk, engineer Peter DuPerry, and law enforcement / military program manager Jeremy Stafford were Mossberg’s main representatives.
Brad Gilpin from Blue Force Gear was there to support our use of their adjustable sling.

A three-shot slug group, shot off-hand by the author at 25 yards. I’ll give credit to the optic and the sling (PC – MuzzleFlash Media).
While not at the event, Trijicon supplied several of its MRO carbine optics for our use with the two 590 pump action models.
Gunsite
Representing Gunsite were Mario Marchman and Paul Garcia, both retired sergeants from California police departments.
After Mario gave a solid safety brief, we got into the meat of the event. Over the next few days, the shotguns would be slung with BFG’s GMT sling. Brad G discussed and demonstrated the use of the sling in conjunction with a long gun. This included its use as a shooting aid and how to control and manage the weapon while recovering and retrieving items from the ground.
I need to credit Mossberg for bringing some of its engineers who had been involved in these projects to the event. That alone reflects extremely well on Mossberg. When we asked for feedback and insights, those people were present to hear them and tell us what had led them there.
The guns we looked at were the 590R and 590RM, the 590 Pro, and the 940 Pro Tactical SPX (which we will cover in part 2).
The Precursor
Mossberg has been involved in a project for an element of an allied nation’s military. The client wanted a purpose-built breaching shotgun with a safety that mirrored that of their issued M4-style carbines. The safeties on those guns were pinned, allowing the user to replace the lever with one of a different length or profile.
The design work for that project carried over into the guns we were about to work with.
The R
What’s the R? It is the model with a rotating safety. Rather than the well-known tang safety mounted atop the rear of the receiver, they have an AR-style rotating safety on both sides of the receiver, located towards the rear of the trigger guard. It works equally well for left- and right-hand shooters. This allows the use of a pistol grip while allowing the shooter to manipulate the safety. The levers are the same length on each side of the receiver. Currently, there is no option to switch to a shorter or differently profiled lever on the receiver’s strong side.
This gun has an 18 ½” barrel with a 6+1 magazine tube. During this event, the MRO was mounted on the Picatinny rail on top of the receiver.
The forward sling mount is on the magazine tube cap.

Both models come with a purpose-designed recoil pad and collapsible stock, Picatinny rails for sights, a Magpul pistol grip, the new forearm, and QD sling swivels.
Some of the barrels had a breaching stand-off, which is seen on the Thunder Ranch models.
The loading port is enlarged and beveled to make it easy to get shells into the magazine tube.
The RM
Next up is the RM. It also has a rotating safety and is fed by a detachable ten (10) round box magazine. The loading port has been modified and reinforced to serve as a magazine well. The magazine is secured and released by means of a lever. If you are familiar with the magazine release for the AK-47 or M14 families, then you know how this one works. If not, you’ll hook the front of the magazine on the front of the mag well before rocking the magazine rearward and upward.
The double-stack magazine is exceptionally stout and rather large—so much so that a traditional tactical reload is off the table. You’ll either shoot it empty or remove it before securing it and then seating the new magazine.
No one who shot these reported any issues with the magazines, either loading or feeding.
Construction
Regardless of the model, the receivers are made from aircraft aluminum. The steel-to-steel lock-up is created between the bolt and the barrel.
A flat trigger has been introduced with these models.
There is also a new forearm with a different attachment system than we are used to seeing. It has a more ergonomic design, with less play between the components. More importantly, it reduces costs.
Additionally, there is a barrel/magazine tube clamp that serves as a mounting point for weapon lights. It is like those found on another manufacturer’s semi-auto.

The R model’s loading port has been enlarged and beveled. Then there is the RM’s magazine well (PC—Mossberg).
Final Thoughts
Even before this event, I have become a fan of Mossberg shotguns in the last several years. While I can modify other products to work better for me as a left-hander, their guns are ambidextrous out of the box. The reliability and longevity of these products in hard-use situations have become readily apparent. Other manufacturers are copying parts of their designs as well.
Evolution is good; not everything needs to be revolutionary. Mossberg has definitely evolved its pump-action shotgun line with the 590R and 590RM models, which have evolved to meet the needs of the end users.