I believe the Sportsman was a variant of either the 11-48 or the Model 58. Maintenance cleaning would involve....
Make sure the weapon is empty and the safety is on and...
Loosen the large cap at the end of the forearm on the magazine tube. Remove, and pull the bbl forward to take off the shotgun. Then, remove the large spring and friction rings,remembering their sequence. Then, clean this tube, springs, and the bbl by the usual means. Reassemble.
Going into the receiver I'd leave to a pro. I suggest taking it to a gunsmith for a thorough interior lceaning and lube. Or.try it if you want to, perhaps your abilities exceed mine. I've slunk furtively into a gunshop with a shoebox full of parts I couldn't get back together once, It's a humbling experience(G)..
As for that "Muzzle Brake", it's probably a Polychoke or a clone. Tightening and loosening it will change the choke. Most had a Compensator built in so it is a Muzzle brake.
Good shotgun, enjoy....
