How does steel case ammo degrade the barrel?
technically its not the steel case as they are usually coated (zinc, poly, or lacquer) to prevent extra wear on the chamber. but the bi-metal bullet can increase wear on a barrel due to the fact it has steel mixed in with the copper, thus making the jacket harder then a normal copper jacketed round. Harder metal causes more friction on the barrel which equals more wear and more heat... at least that's how I understand it.
But I have noticed the lacquered cases tend to get stuck in ARs after the barrels heat up lacquer gets soft then hardens it can take some really muscle to get them unstuck.
Personally If your looking to shoot on the cheap its a non issue. The amount of ammo you'd need to buy and fire is FAR over the cost of the rifle and you can always re barrel a gun.