Is this a Sig 556 design flaw?

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geojap

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I was installing Sig 551 handguards today on my Sig 556 and noticed something that did not seem right to me. I thought I would ask and see if anyone else can replicate this behavior. The bolt hold open is releasing and allowing the bolt to slam shut when I extend the stock to the fully extended position.

First, my rifle. It is a stock Sig 556, purchased from Bud's Gun Shop. The serial number is 57xx. After I received the rifle, I have installed a Magpul CTR Commercial stock, Leupold QD rings, a Sightron SII Big Sky scope, and Sig 551 handguards. I have fired ~500 Silver Bear rounds through this rifle without any malfunction.

I found out today that when I move the stock from the compressed position, to the fully extended position and let the buttsock hit the end of the tube, the bolt hold open will release the bolt and it will slam shut. Is this normal? I would not expect this to be much of a problem unless one had a broken firing pin, but it still seemed highly unusual and did not function the way that I was expecting. Can anyone else replicate this behavior in their Sig 556?

I'll post pictures below. This is my 556:
sigrifle.jpg


This is the rifle, just before I extend the stock. You can see that the bolt is open and the stock is compressed. I am not touching the bolt.
open.jpg


This is the rifle after I have extended the butt stock with my right hand, not very forcefully at all, so that it has fully extended and has contacted the end of the tube. You can see the bolt has shut. I have not touched the bolt or bolt release catch.
closed.jpg


Granted, this is a Magpul CTR stock. The Sig stock should function in a similar fashion though. I have not tried this with the Sig butt stock yet. Can anyone else replicate this? I don't consider this especially serious unless I happen to break the firing pin, have a loaded mag in the mag well, and happen to extend the stock all at the same time. But it still surprised the dickens out of me when the bolt slammed home the first time I extended the butt stock. I can post more pics or explain something in more detail if there was not enough explanation included here already.

Let me also say I'm not bashing Sig, but asking questions here. I'm a huge Sig fan actually. I own a P229, P239, and have a P225 on order from AIM right now. This Sig 556 is the best shooting semi-auto rifle I own, along with my Tromix .223 Saiga. I think I'm going to post this over at the Sig Forums also. Thanks for reading.
 
Seems like a serious problem to me. I'd say you have to swap out the stocks and see if it does the same thing with the OEM parts.
 
Spyvie, you'll give the youngster a complex.

This is not an issue unless you make it one. Only open the stock when the bolt is forward, and then commence to charging the weapon.;)
 
you'll notice that with any AR as well, you hold the bolt to the rear then tap the buttstock, like doing the 15 count manual of arms, and it'll send the bolt forward, I've had M16s that you practically breathed on them and the bolt would go foreward, but those were old worn out basic training rifles, but even brand new rifles will do it, just not to the level of sensitivity.
 
Ok, sounds good. Thanks for the explanation and relieving my mind that there might be something wrong with the rifle. Having never owned an AR (but quite a few FALs and AKs), I've never experienced anything like this before.
 
TopGunSupply gets them in periodically. Only like $92 shipped. You can sign up for an email alert and be notified when they get them in.

I had heard that they completely change the way the rifle handles, but I was skeptical. Well, I'll be darned if it isn't totally true. The 551 guards probably weigh 1/3 of what the 556 hand guards weigh. It is actually a pleasure to heft the rifle up to your shoulder and get a cheek weld now.
 
Try having the stock in its collapsed position, charge the bolt and have it held open, then smack the stock with your palm.

If the bolt slams forward then it has nothing to do with the stock, rather the bolt release might be to loose/weak.......or it might be normal.

I don't really see how the movement of the stock affects the bolt or bolt catch directly. It must be due to shock.
 
You're right. If I smack the butt hard with my palm or drop it on the butt from about 8 inches up, it will release the bolt then too.
 
Well that sure is interesting.... my Sig556 is plain stock serial number range 7XXX and does the same thing. Not every time the stock is extended but probably half the time, just tried it probably a dozen times and it kinda seems to be a bit random. Half the time it slams shut if I extend the stock forcefully and the rest of the time it doesn't. A softer extension of the stock is about the same, sometimes it does sometimes it doesn't. Slapping the side of the receiver with the bolt held back doesn't ever seem to release the bolt but a firm whack to the butt will cause it to release most of the time.
 
Thats not a problem unless you consider it a problem that every AR15/M16 will let the bolt go if you smack the butt stock on the ground. Nothing wrong with the weapon, not a design flaw, its just the nature of springs and metal.

An object in motion wants to stay in motion, smacking the butt of the weapon is moving the weapon, and the bolt wants to keep moving... enough so to let the bolt catch release. Its not a design flaw, just physics.
 
I found out today that when I move the stock from the compressed position, to the fully extended position and let the buttsock hit the end of the tube, the bolt hold open will release the bolt and it will slam shut. Is this normal? I would not expect this to be much of a problem unless one had a broken firing pin, but it still seemed highly unusual and did not function the way that I was expecting. Can anyone else replicate this behavior in their Sig 556?
Click to expand...
This doesn't have anythig to do with the firing pin (enclosed as it is inside the bolt) but is a function of the bolt hold open (BHO) lever thingee being jarred enough to drop down and let the bolt return to battery.

My 556 does this, as do all of my ARs if I handle them roughly. It's not a problem; just a characteristic of the weapon.

Leave an empty mag in the weapon, and you'll notice that it takes a lot more force to get this to happen. That's because the mag follower will hold the BHO in place.
 
Thanks for the heads-up on the 551 stock.

I didn't fully read the initial post but having done so will try to give my explanation to add to the others.

As with most semi and full auto guns that enable the platform to hold the bolt back either by the shooter or the magazine actuating their automatic bolt stop system. The small latch that hinders the bolt from moving forward is usually spring loaded to keep it out of the way of the bolt when it is cycling. Manually moving the bolt stop latch by hand or by the magazine follower overcomes this spring and causes the stop to extend into the path of the bolt holding it open and stopping it from going home. When the bolt is moved back either manually or by a jolt from the rear, the pressure of the bolt caused by the recoil spring pushing it forward is removed and the spring loaded bolt (no longer experiencing the pressure from the bolt) stop moves out of the way allowing the bolt to move forward.

To try to describe it further, the bolt is normally seeing force from the recoil spring pushing it forward. As the bolt cycles, with either the shooter manually actuating the the mag follower pushing the bolt stop latch, the latch protrudes into the path of the bolt when the bolt is near or at its rear most position. As the bolt goes forward, it meets the latch and stops. The latch is held in place by the force of the bolt (applied by the recoil spring) against it. The latch is usually under its own spring pressure that would normally hold it down during firing. However, the spring pressure for the latch is small enough that moving the latch with your finger or the mag follower is enough to overcome it and allow the latch to extend into the path of the bolt when required to.

When the bolt is moved back slightly, it relieves the pressure it is applying to the latch and the latch (under spring pressure of its own) moves out of the way. How the bolt moves rearward can be done in a number of ways. You can manipulate the bolt manually or the gun can get a tap from the rear. The bolt itself is not a stationary part directly attached to the receiver. It needs to move back and forth and rotate and/or pivot to do its job. It is basically a spring loaded weight. When you bump the rear of the gun, the inertia of the weight moves back against the spring which was pushing it forward against the latch. With this pressure removed, the latch moves out of the path of the bolt.

This does not include guns such as HK G3s/91s etc. (except MP5/10s/40s, etc.). The bolt can also be released by actuating the latch out of the way manually.
 
gosh gave me a heart attack reading the title and fp. i have a 556 swat coming in and it has a magpul ctr from the factory.
 
Rtfm!

Sorry to chime in so late, but I had a question about the buttstock assembly and looked the SIG 556 up on Google.

May I quote from the Fabulous Manual:

CAUTION: Dropping or jarring the rifle with the bolt locked
open can cause the bolt catch lever to disengage. If a magazine,
loaded with ammunition, is inserted in the rifle when this occurs,
a round will chamber with the potential for an unintentional
discharge. ALWAYS handle the rifle as if loaded whenever a
magazine is inserted in the rifle. ALWAYS place the safety lever
in the “S” (SAFE) position and unload the rifle whenever
shooting is complete.

**

I might add that none of this happens if you have a properly-functioning and EMPTY magazine in the rifle at the time. I wouldn't recommend that as a habit, because you can suddenly find the rifle loaded after some joker replaces your empty mag with a charged one...:cuss: Some things should NOT be joked about with.

Read The Fabulous Manual!
 
My sig 556 does the exact same thing.

I did notice that with an empty magazine inserted, it doesn't.

I think i'll pass on testing this with a full mag tho :D
 
Before I sold my Sig 556 I noticed that it did the same thing. It had the factory folding collapsible stock on it.

Wasn't sure if it was a design flaw or not but it never really bothered me and it doesn't bother my friend either who I sold the rifle to.
 
This reminds me of the customer who came in off of the range one day, all upset at me because his brand new XD-M was "jamming" on him.

"After I shot the last round, that slide thing got stuck in the open position and I can't get it to go forward! This thing is a piece of crap, and I'm mad as hell!"

If people would actually RTFM for once, and not assume every little detail is a massive design flaw, there'd be far fewer threads floating around the interwebs.
 
Totally normal for this type of weapons design. When I was on active duty I actually did it on purpose to chamber rounds once inserting a fresh mag in the well. I found I could load and tap the butt of the rifle while on the run and didn't have to "feel" for the bolt catch button.

when I was in basic I actually was issued a rifle the had the "button" on the mag catch broken off. I spent the entire 2 months attempting to get the company armorer to fix it but it never happened. Guess what technique the range NCO's told me to use? Yep, tap the butt of the rifle with your hand and shoot.
 
We bought a new Sig 556 classic last week. Noticed what i believe are two design flaws. This model has the foldable butt stock that sticks to the Swiss style handguards with the aid of a 'nipple' that inserts into the stock and keeps it in the folded position when desired.
However, if you want to use the weapon with the stock folded, you can't release the magazine when it's spent--because the folded buttstock now covers the mag release button!
We noticed on older Sig 556 models the mag release button was lower down. Why didn't the leave it that way for the newer models with the foldable buttstock, and it would have worked perfectly?
That's a shoddy design feature.
Also, continual ejection of spent magazine cases will mark the left side of the 'mouth' they are discharged though, wearing the Teflon finish, or whatever finish it is. The gun store where we bought from said this is a 'common prob' with the Sig 556 and variants of the model.
If it's a common problem why haven't Sig fixed it by now by either widening the ejection 'mouth' or having a thicker coating finish on the area that easily marks?
We were disappointed with these two problems, and expected more from a supposed high-end rifle from Sig.
Anyone feel the same way?
 
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