It's the Remington Model 58, which was Remington's first gas-operated semi-auto shotgun.
The Remington 11-48 was a good guess, but it used the same Browning long-recoil system as the Model 11 and Auto-5 and that really sticks out when you see the patent drawing for it.
My late father wore the bluing off his Sportsman 58 he shot so many bobwhite quail with it. Of course, that was back when there were quail in the Southeast.