A company called Hoplite Armor has come up with an armor package that are shoulder plates, that covers about half of a shooters bicep. The system is attached to the arm via a strap that connects to an existing plate carrier shoulder straps, and then another strap that wraps around the arm of the shooter. Ilikegunz has a pretty decent review of them, to include shooting them with 5.56 and 7.62, the plates stopping both of the calibers. They are selling for $400, while the actual carriers are going for $100 on the Hoplite Armor website. The carriers can fit both types of plates. It also has a Velcro section so patches can be added to it.
Now, of course this is going to get into the whole slick versus armor argument, or minimal versus maximizing armor. I know that a lot of SWAT teams already use shoulder armor, but those are probably much more costly than this more civilian alternative. The military has already gone through this uparmoring process from Iraq, where the Army extensively used shoulder, neck, and groin armor because of the extremely built up areas where the majority of combat operations took place in. Case in point, in that using shoulder armor, it is very situationally dependent and probably best for extremely tight areas where multiple threats occur. But as for open terrain, I think it would be hard to justify this as adding more armor slows and wears a soldier down, considering the amount of movement that takes place.
From the company website-
At a weight of only 1 lb. per plate the Hoplite HASP (Hard Armor Shoulder Plate) is a unique Level III design unlike anything currently on the market. The HASP System provides extended coverage to the upper body and covers the vital organs, protecting the operator from lateral impacts. The HASP (Hard Armor Shoulder Plate) is fabricated according to the highest quality standards, using the best polymer based materials available. The HASP (Hard Armor Shoulder Plate) will fit in the standard DAPS carrier or in HASP Plate Frame. ONE SIZE Sold in Pairs. *Shown with HASP Frame Carrier (sold separately).