High volume shooters buy and shoot bulk ammo by the 1,000s of rounds. The time and cost of reloading for these users makes reloading some what prohibitive. Buying steel case ammo in 10,000 round lots can keep the cost of high volume use very low. Even small lots are much cheaper some times below reloading cost. I am not aware of over size chambers ? over spec will rupture cases regardless? I have seen and used 1,000s of steel cased rounds and never seen all these problems in modern sound firearms.