...FYI...some additional technical information for you...i hope that it provides a little insight on the operating system of your 556...
Posted: Tue Apr 07, 2009 11:25 am Post subject: Carrier Issues
...lately there has been some concern regarding the bolt carrier group (BCG) *hanging* over the hammer...as you "very slowly" bring the BCG fowards while holding onto the charging handle...this ***in itself*** usually will not prevent the weapon from functioning normally...and will *usually* dissapear after you fire and continue firing the weapon...smoothing out the rough surfaces that "many" new weapons may have due to the manufacturing process....this surface roughness induces additional friction into the operating system which "robs the operating system" of some of the energy required to cycle the weapon "optimally"...
...the 556 is a self loading gas driven reciprocating design...as the bullet passes the gas port...the high pressure gases ...following the path of least resistance.... flow through the gas port ( at this very instant only...the gas system is charged with a volume of gas at it's *peak pressure* level...referreed to as "peak port pressure"...which is "not the same" as "peak chamber pressure"...peak port pressure is dependant on the location of the gas port relative to the chamber...any residual "dwell" pressure remaining as the bullet is still traveling through the barrel will be less than the peak"gas port" pressure level "that drives" the operating system...
...the gas then flows through the gas block...through the gas port in the gas valve...through the piston tunnel and into the gas cup of the gas valve...the high pressure gas...exerts "force" on anything containing it...and again follows the path of least resistance...forcing the piston reawards...the only movable component it is pressing against...as the piston moves rewards in it's stroke a very short distance...the gas is vented and is no longer exerting any force upon the piston to drive it rearwards...at this point the operating system is *on it's own*...it "will not be further assisted" in any way...the total energy initially imparted upon it will decrease as the operating system uses this energy to drive itself...either through mechanical disadvantage or doing work...for example...the locking..."unlocking" and camming of the bolt in the carrier... the BCG cocking the hammer against the force of the hammer spring...and again when the BCG pushes the hammer down against the force of the hammer spring as it passes over the hammer...likewise...pushing down the rounds in the magazine while going reawards...and striping a round from the magazine as it comes fowards...further energy is also lost through any "friction" present in the operating system...the bottom line is that the "net (total) energy" to the carrier (the piston is attached to the carrier) must be great enough to over come the resistance of the recoil spring...allowing the BCG to travel rearwards far enough to compress the recoil spring sufficiently...so that the recoil spring becomes "charged" with enough energy to sucessfully pull the BCG fowards with enough force (energy) to push the hammer down against the force of the hammer spring....continue forwards and strip a round from the magazine...chamber it...force the extractor lip over the rim on the case head...allow the bolt to cam in it's track forcing it to rotate and lock into battery...keep in mind that the reward travel of the BCG is "powered" by the energy initially transfered to the carrier by the high pressure gases...the foward travel is "powered" by the energy *stored* in the recoil spring...due to it being compressed...compression springs "store energy" to be later used to perform "work"..in this case to drive the operating system...pulling it fowards...this is a very basic explination of how the system works...there are other factors that will influence the efficicency of the system...such as the "newness of the weapon"...the relative "tightness" of the components" the state of lubrication / type of lubricant...even the tempture of the components...which may have an effect on their dynamic co-efficient of friction (net induced friction) by affecting the demensional relationship with the other components that they have a meachanical relatiionship with...
...with all of this in mind...there are several factors that can contribute...allowing the BCG to "hang" over the hammer while very slowly bringing it fowards...some of which are...a weak or bad recoil spring...which just isn't pulling hard enough as you find "just that right spot" where the hump on the bolt shaft contacts the hammer...allowing the BCG to stop there...another contributing factor could be a very tightly wound hammer spring...or a rough hammer axel...and / or a rough hammer axel tunnel...inducing just enough resistance to prevent the hammer from pivoting freely...there could be a little roughness on the hammer where the hump of the bolt contacts the hammer as it goes fowards...kinda acting like sandpaper...just enough to hold it there...or a combination of these working together...and there are others...as the weapon cycles normally...due to the BCG's velosity...it has a considerable amount of energy...and will simply continue to go fowards...so that this usually is not an issue and will work itself out...as long as it is not a *symptom* of a bad recoil spring ...in which case the spring cannot "store" enough energy to accomplish ***all*** the work that needs to be done to operate the weapon *optimally*...then you may have malfunctions...
...***if this "hanging*** is symptomatic of a "bad / weak recoil spring"...then "no amount" of cycling or firing the weapon will eliminate it...and if the recoil spring is really "bad"...you will certainly have malfunctions..
...a mountain has been made out of a molehill here...this is something i would not be too worried about...just clean and prepare your weapon properly...and use good ammunition and good mags and shoot her...as i have said many times in the past...it's not your place to gunsmith your 556...if you are having malfunctions...and these malfunctions keep occuring after firing a few hundred rounds...and you are using good mags and good ammunition...it's time to let Sig sort it out...
...BTW...there are mags and then there are mags...the market is flooded with many types from many different manufactures...in my opinion there are ***only*** two types out there that can be depended on...especially when you are troubleshooting your weapon...
...any USGI NATO standard magazine with the standard spring and anti-tilt follower (green)...or better yet with the Magpull anti-tile follower installed...an example of currently avaiable mags are the D&H mags...
...the Magpul P mags...even though aftermarket...they work
...i personally wouldn't consider any of the others...including HK mags...Lancers...the SIG mag...none of them...no exceptions...
...i don't feel i've covered this in nearly enough detail...but hopefully i've covered this material well enough to give some of you a slightly better understanding of your 556...and some of the influencing factors that can affect the efficiency of the operating system...in a properly assembled weapon that meets the print specifications...the systems worst enemy is "friction"...that's why it's important to properly lubricate the weapon with a good lubricant...and also to keep her "relatively" clean in the more critical areas...
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http://www.sigarms556.com/viewtopic.php?t=5637
