The M16 service rifle and the M4 carbine are well known.  Although these firearms have proven generally satisfactory, the M16 and M4, as well as other firearms, have a variety of reliability shortcomings.  These reliability shortcomings can result in a malfunction of the firearm.  These reliability shortcomings are becoming more evident as the use of higher capacity magazines increases.  Such malfunctions can have serious consequences and are thus highly undesirable.  Some of the reliability shortcomings are discussed below. 
Gas operated firearms use some of the gas from a cartridge being fired to extract the spent case of the cartridge and to chamber a new cartridge.  The gas travels from a port in the barrel to a gas cylinder where the gas pushes a piston within the gas cylinder to operate a mechanism for extracting the spent case and for chambering the new cartridge.  In some firearms, such as the Ml 6 and the M4, the gas cylinder is formed in the bolt carrier and the piston is part of the bolt.  In such firearms, gas is provided from the barrel to the gas cylinder by a gas tube. 
In other firearms, such as the HK416, a separate (not part of the bolt) piston is used.  The piston is disposed in a gas cylinder that is not part of the bolt carrier.  This separate piston applies force through a tappet or operating rod and a bolt carrier to operate the mechanism for extracting the spent case and for chambering the new cartridge. 
Whether or not the piston is part of the bolt, it is desirable to prevent gas leakage between the piston and the cylinder.  Contemporary gas operated firearms commonly use a plurality of piston rings which fit into a groove of the piston in an attempt to provide a gas seal between the piston and the cylinder to mitigate gas leakage.  For example, the M16, M4, and HK416 use three rings.  Each of the rings is a split ring that has a gap formed therein to facilitate installation of the ring and to allow the ring to apply an outward spring force that tends to seal the loose fit between the piston and the cylinder. 
Contemporary rings possess inherent deficiencies which detract from their overall effectiveness and desirability.  For example, the gaps of the three rings occasionally line up in a manner that allows hot gasses to flow readily through the gaps and thereby undesirably bypass the rings.  When the hot gases flow through the gaps, the force provided by the gases to extract a spent case from the chamber and to chamber a new cartridge is undesirably reduced.  Further, when the hot gases flow through the gaps, the hot gases can burn the ends of the rings and thereby undesirably enlarge the gaps.  It is desirable to provide rings that mitigate undesirable gas flow thereby. 
Contemporary gas tubes possess inherent deficiencies which detract from their effectiveness and desirability.  For example, contemporary gas tubes can overheat and lose strength, particularly during sustained fully automatic fire of the firearm.  The higher level of heat associated with sustained fully automatic fire can result in undesirable thermal expansion of the gas tube both radially and longitudinally.  Such thermal expansion can be substantially beyond an amount that can be accommodated by the available space in the firearm.  Such thermal expansion can result in sliding/clearance fits becoming interference fits.  That is, a sliding fit can undesirably become a non-sliding fit, i.e. can freeze or jam.  When the gas tube heats up excessively, the weakened and expanded gas tube can bend and be damaged because it is not free to slide, thus causing the firearm to become inoperative.  It is desirable to provide methods and systems for mitigating overheating in gas operated firearms. 
Forward and rearward bouncing of the bolt carrier can cause the cyclic rate of a firearm to increase substantially.  This increase in the cyclic rate can reduce the reliability of the firearm and can increased wear on the firearm.  It is desirable to provide methods and systems for mitigating both forward and rearward bouncing of the bolt carrier. 
The gas port of a contemporary M16/M4 firearm is subject to erosion caused by bullet scrubbing and propellant bombardment.  Such erosion results in enlargement of the gas port and consequently an undesirable increase in the cyclic rate of the firearm over time.  M4 carbines, which have the gas port located at a rear band of the front sight, are particularly susceptible to such erosion.  This undesirable increase in the cyclic rate can eventually result in malfunction and damage to the firearm.  It is desirable to provide for the placement of the gas port and metering of gas in a manner that does not result in an increased cyclic rate over time. 
Gas operated firearms, such as those of the M16/M4 family of firearms, have bolt and barrel locking lugs that secure the bolt to the barrel during firing.  Failure of the locking lugs can result in the firearm being inoperable.  It is desirable to provide more robust locking lugs for such firearms, so as to mitigate the undesirable occurrence of failure. 
The cam of a bolt carrier of such firearms cooperates with the cam pin of the bolt to lock and unlock the locking lugs.  In instances where pressure of the gas system has increased and the cyclic rate has consequently also increased, such as due to erosion of the gas port, the cam can cooperate with the cam pin to attempt to unlock the locking lugs too early in the firing cycle.  In this instance, the gas pressure in the chamber can be too high to allow the locking lugs to rotate fully.  When this happens, one or more of the locking lugs can break.  Again, this can result in the firearm being inoperable, thereby potentially resulting in loss of life in situations such as during police use and battlefield operations.  It is desirable to assure that the gas pressure in the chamber is sufficiently low to allow the locking lugs to rotate fully when the bolt is being unlocked. 
These reliability shortcomings of such contemporary firearms can result in the failure thereof.  The failure of the firearm, particularly during critical police use and battlefield operations, can result in loss of life.  Therefore, it is desirable to provide firearms that do not suffer from these reliability shortcomings.