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my sig522 quad rail fits my 556. as long as you don't have the improved rail where it's attached to the top rail of the receiver then it should interchange.
 

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My 556 Classic SWAT rail has an overhang which slides into a corresponding notch in the forward portion of the 556 receiver's upper rail. The 522 doesn't have this notch on the upper rail and is instead flush with the forward portion of the receiver. You will need to accommodate for the overhang on the 556 SWAT rail in order for it to fit on the 522. This can be done by sliding the oval assembly which houses the fake piston tube forward. It is fixed to the barrel by a hex screw and is easily shifted about 2 mm towards the muzzle. You can then affix the 556 rail but it will still require a bit of a squeeze as the lateral dimensions of the 522 are not an exact mirror of the 556. It will work, but the fit will be tight and you'll loose some finish in the process.

I ended up taking the 556 rail off the 522 after a day. I still may switch it again. It's nice to get a few ounces off the 556.
 

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I picked up a new 522 at a local gun shop today for $420+tax. Good price so I jumped on it.

I wanted to see if the Sig quad rail from my 556 Classic SWAT woudl fit. They do but as mentioned above they do require a small bit of milling. Fortunately I have a benchtop 4 axis mill in my shop and proceeded to try it. Took about 20 minutes but with a small amount of milling on the 556 rails they fit the 522 no problem. The modification does not affect fitment on the 556 so I can put them back on the 556 if I choose. (and I likely will).

As was pointed out, the fake gas block needs to be moved forward about .090" which is easy since it's held in place only with a set screw with 3/32" Allen.

Some pics after the modification below













 
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