GMerlon is correct but if you happen to experience a soft head failure the entire rifle is probably going to be wrecked and you are heading to the ER or undertaker.
Example of a soft head failure I investigated. This happened on the sixth round in a M1 Garand. I was contacted and told him to send me all the fired cases prior to it and he did. Headspace was perfect on the first 5 and no 6 completely wrecked the Garand.
This round was loaded in Britain for the Greek Army in the 50s and found out this arsenal had these prior. The bolt lugs held, the receiver held but the escaping gas broke the lower stock in about 5 pieces.
Soft heads occur when the cases go through the annealing line upside down and the flames are directed at the head of the case instead of the neck/shoulder and is one of the bad things that can happen on a manual loading line thus if you ever see a round of military ammo where the head is blue and the other ammo the neck/shoulder is blue this is your warning do not fire it under any circumstances.
This was known to occur on 5.56 ammo loaded on manual lines at LC, TW etc and we got a M16 in that had sustained a case failure about every four months when I worked in the Product Engineering section for the Army Small Cal Lab at Picatinny Arsenal. The upper receiver is bowed outwards and is ruined. We never had a case of lug faillure on the bolt or barrel socket. It was estimated that this occurred once every 30 million rounds..