ATI has some products to make your single shot Rossi or H&R rifle look more like an AR-15 patterned rifle.
The above photo has the “Collapsible, six-position SHOTFORCE Buttstock with Pistol Grip and Forend Package is patterned after the Colt AR-15/M-4 style assault rifle.” installed as well as the Muzzle Mount, laser and heat shield. The Muzzle Mount is a fake muzzle break and does not extend past the end of the barrel.
Now I have no problem with people installing a pistol grip. If you prefer it ergonomically then go for it. But a head shield on a single shot rifle?