For anyone interested. From Armalite's site.
January 4, 2002
Technical Note 49, Cartridge Case Failure in the M16 and Similar Rifles
We recently inspected an M15A4(T) rifle that suffered a catastrophic failure. The owner of the rifle in question had hand loaded match-grade cartridges with a common 69 grain bullet. He believes that there was something wrong with the rifle.
Historical evidence would indicate that the owner is incorrect. Almost all catastrophic failures of M16 class rifles are due to excessive pressure far above the original design intent. The excessive pressure can have any of several causes including: defective cartridges (bullets too heavy, too much propellant, inappropriate propellant, incorrect cartridge headspace, lubricant on exterior of cartridge case, cases reused too many times, etc), or bore obstructions ( including dirt, sand, water, or a bullet in the bore from a previously fired defective cartridge). Only a small percentage of failures are attributable to breakage of a weapon component-------usually the bolt.
The purpose of this Technical Note is to describe the typical sequence of events for the information of maintenance personnel.
Sequence of events:
M16 rifles, and all similar models, suffer damage in a characteristic sequence. Fortunately, the design of these rifles provides considerable shielding that contains the pressure and residue, and prevents injury of the shooter. Army and ArmaLite records reveal no significant injury in any incident, a truly outstanding record.
When a normal cartridge is fired, pressures inside the cartridge case rise to 55,000 pounds per square inch or more. Defective cartridges or bore obstructions will cause pressures to rise dramatically.
Upon propellant ignition, the interior of the cartridge case is pressurized pretty evenly. As pressures rise the case walls expand and seize the sides of the chamber. The case stretches rearward until the base of the cartridge contacts the bolt face which supports it, putting pressure on the bolt.
A cartridge case failure allows this high pressure gas to escape into the barrel extension, where it flows into contact with the bolt and bolt carrier. As it expands, the gas has a larger working surface to work on than it does inside the cartridge case. This places tremendous stresses on the bolt and carrier.
The high pressure gas bends the front end of the extractor outward, locking it behind the locking lugs of the barrel extension. In rare cases the barrel extension may be split.
The gas strikes the face of the bolt. The energy deposited in the bolt can cause the bolt to split along a line running between the extractor slot and the firing pin hole, shear the lugs from the bolt, break the bolt at the cam pin hole, and/or break the tail off the bolt. Not all of this damage is normally seen on a single bolt.