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As Francis said, the problem with steel cased ammo is that the cases wont seal reliably in the chamber. There are some rifles, due to design characteristics, that will work safely and reliably with steel cases but the M1A isn't one of them. If the case doesn't seal properly, not only does it create inconsistent pressures, but the combustion gases can also escape in to the receiver. Depending on how much leakage there is between the case and the chamber walls, you can just experience a really dirty bolt and receiver or you can get a face full of hot gases.
Hornady created the steel cased ammo because it is cheaper. They got around the gas sealing issue by covering the steel cases with a polymer coating. The polymer is designed to ensure a good chamber seal. The military is also looking in to developing steel cased ammo for the same reason.
I almost forgot to mention, that steel cases also increase the bolt face pressure since the case slides backwards.
Hornady created the steel cased ammo because it is cheaper. They got around the gas sealing issue by covering the steel cases with a polymer coating. The polymer is designed to ensure a good chamber seal. The military is also looking in to developing steel cased ammo for the same reason.
I almost forgot to mention, that steel cases also increase the bolt face pressure since the case slides backwards.
