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Old March 5, 2013, 01:57 PM   #1
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SIG-556 question.

I own a SIG 556 Rifle, have owned it for the better part of a year now but havn't gotten to shoot it very much only about 200-400 rounds.

I noticed something odd and thought I should consult the "experts". While inspecting the rifle, unloaded with bolt locked back any kind of real disturbance of the rifle, like bumping the stock even lightly against my gun cleaning table will cause the bolt to "release" and slam closed.

This does NOT happen when an empty magazine is inserted and I have not tried with a full magazine inserted. It is repeatable about 100% of the time.

Is this normal for a SIG-556? I've never tried to duplicate this in my LGS or anything but I would like to know if I have something to worry about.
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Old March 5, 2013, 03:21 PM   #2
Pistola Pedro
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Nothing to worry about

Can't say that mine has done that a whole bunch, but I can recall it happening if I give it a smack on the butt pad with no mag in. Hurts nothing and is no sign of anything being wrong.
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Old March 5, 2013, 05:57 PM   #3
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Glad to hear that, appreciate the reply!
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Old March 5, 2013, 06:51 PM   #4
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It doesn't take much to release the bolt on a 556, when I reload mine, I can usually get away with just smacking almost anywhere on the side of the rifle to chamber a round. Much less precise than my AR.
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Old March 6, 2013, 03:39 PM   #5
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The SIG 556 and pretty much any other semi-auto rifle that has a last round bolt hold open is going to close the bolt if you bounce the butt with no magazine inserted. The bolt hold open is under spring pressure as well as the bolt. If the bolt is pulled back with the bolt stop engaged the bolt stop will drop under spring pressure. Depending on the spring weights and weight of the bolt will depend on how easily the bolt will drop.

The reason the bolt does not close with the empty mag inserted is because the mag follower is pushing up on the bolt hold open to keep it engaged with the bolt. If you do this with a loaded magazine it will strip off a round and chamber it making the gun ready to fire.

The amount of force to get the bolt hold open to drop will vary and is dependent on spring pressures and the lubricity of the two surfaces.

You should check your bolt release and see if it operates to its full travel by the mag follower. If it doesn't try a different mag or lubricate it to see if that helps the travel or manually move it up and down. Just flip open the receiver and insert the mag and observe the travel. Maybe a weak mag spring as well.
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Old March 7, 2013, 11:50 AM   #6
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This is normal, when there is no empty magazine installed, the lock is not being forced up, the lock is actually spring loaded to the down position so the empty magazine must overcome this spring to force it up in the first place.
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