We all know about the numerous different types of QC problems documented about the Sig Sauer 556. Here is one I discovered while disassembling a 556 made in 2009, and I don’t believe anyone else has brought it up before. I chose to post it here at M4carbine.net, since this forum’s users take their weapons more seriously so to speak than forums whose member base consists mainly of casual/recreational shooters.
First, the function of the gas valve on the 550 series needs to be briefly explained. The valve has two positions, one for normal shooting and one for when the weapon is fouled (or frozen with ice) to the point of not cycling. This second position is also sometimes referred to as the “Emergency” setting, and lets much more gas in to act upon the piston to dislodge. The gas pressure in this position is so strong, the manual (Both US and Swiss) caution NOT to run the weapon in this position unless absolutely necessary due to the added stress on the parts.
From the Swiss manual:
The valve has two ports, the smaller one for the normal setting and the larger is obviously for the fouled/emergency setting. On both sides of the gas block, there are downward venting exhaust ports (Located between the roll pins) :
The smaller gas valve port is directly underneath and in line with the exterior rib, which during normal use is in the vertical 12 o’clock position. The smaller port lets in less gas from the barrel into the system, and at the same time the larger gas valve port will exhaust excess gas out of the left side of the weapon. Switch the rib to the emergency setting, and the larger port is now inline with the barrel. Obviously the larger hole lets in more gas, but the smaller port now does exhaust duty out the right side.
So its not just a matter of having small or large inlet ports to choose from for letting gas in from the barrel, but you also have a smaller and larger exhaust port as well. And the only choices of this combination are “Small intake, Large exhaust” or the brutal “Large intake, Small exhaust.”
Here is a photo I saved that was posted about a year ago on another forum of a Swiss gas valve. I do not remember the poster’s name, but if you are here shout out and I’ll give you due credit. Notice the normal (Small) port on the left, larger on the right.
And here is what I found when I compared my own Swiss 551 valve (left) next to the US made 556 valve (on the right) :
I don’t have access to any tools or even drill bits small enough to measure the port sizes. One person I showed the photo to thought it was somehow an optical illusion, so I verified the size difference with a sewing needle.
Obviously I think it would probably be a good idea for 556 owners to check their valve ports before the excess wear and tear of unknowingly shooting in the “Emergency” setting takes its toll.