MP5A3s (possibly MP5A5s, but MP5A3s seem more overall popular with law enforcement, and full auto seems more like what USSS would choose.). I’ve never seen MP5Ks in USSS hands, but I wouldn’t be surprised to learn about them.
Remington 870 PGO shotguns. Probably intended for breaching, but possibly for less-than-lethal rounds. The USSS agent in the photo is carrying 5.56mm magazines, making the shotgun clearly not the primary long weapon.