Having seen a 1918 Lithgowicon #1MkIII* receiver with the charger bridge removed it would be very easy to crack one at a rivet hole. This is because there is very little meat around the rivet holes on the left side of the receiver.

The use of carbon steel is some what vague since it does not imply what % of carbon.

I continue to be puzzled about failing bolt heads.
A properly manufactured bolt head is like a small anvil. It is solid and it is under compressive forces. I can understand if a projection breaks off but the solid body of a bolt head would be very difficult to damage without breaking the bolt body.