Hello, this is my first post and it saddens me that it has to be a complaint. I have several Ruger rifles and pistols. I love them all. But, my AR556 doesn't seem to like to feed the cartirdges into the chamber. What is happening is the bolt is not closing completely. Then the cartridge is stuck in the chamber. The rifle is new with less than 100 rounds through it. It has been cleaned, several times and oiled very well. I ran about 60 rounds of Magtech through it when I first got it. I came across a good deal on some Geco, so I bought along with some Armscor. I managed to get about 18 rounds of the Geco through it before things started to go wrong. Misfire with the cartridge stuck in the chamber. Once that was removed, I tried again, it fired but the next round got stuck without being fired. Got that out, switched to the Armscor. Can't even get the bolt to close with those. Cleaned the chamber and neck again, no issues there. So, I got my calipers out and started measuring the rounds. The magtech all consistently measure 9.41mm at the widest part of the body of the shell... All the others consistently measure 9.48-9.51mm at the same point. I got to looking at some specs online for those cartridges, and what I found is that the max dimensions of those cartridges are supposed to be 9.55mm. So, if my ammo is within spec, is it possible that my chamber ( neck) is too small?? I removed the upper and dropped one of the Geco rounds into the neck and noted how far it fell in on its own. Then I put a magtech round in there and noted how far it went on its own. The magtech went significantly further into the neck than the Geco with nothing other than its own weight. Nothing is more frustrating that a new gun you can't shoot!! BTW, after all the issues with the Geco and Armscor ammo, I went back to the Magtech and have no issues.