Just FYI, if you are looking for a 1913 adapter to put a side folder etc on your 556XI, CDNN has an awesome deal on the adapter from Sig. I had been looking for one to add an MPX side folder PSB but they were about $90 and while I waffled sold out everywhere. Sig website showed they have been removed them from the site.

I lucked out and found them at CDNN for a blowout price, $29.99, ended up being $39.98 with shipping. I bought it Thursday night and it shipped Friday, showed up Monday.

Works good so far on my 556XI PSB pistol, set up right handed the bolt probably won't cycle with the lefthand MPX folder I put on folded all the way, and the receiver extension knuckle hits the sling mount when folded all the way which I'm still trying to sort out (remove it, flip it, or leave as is) but it works solid of you want a 556XI side folder. I'll probably buy a KAK Shockwave blade and buffer tube to replace the Sig brace. If you want one I recommend grabbing one soon, I believe Sig has discontinued manufacturing these.

https://www.cdnnsports.com/sig-sauer-psb-stock-adapter-pic-rail-xi-556-552.html#.Wuk2-8tMEwD

I saw those, What stock fits the rail?  Is it for XI only?

Any MCX/MPX stock.

Thanks,  I just bought a 1911 holster.  Got a deal on a Mitch Rosen holster.

Bag it came in says "PSB BUFFER TUBE ADAPTIVE KIT, 1913, FOR XI, 556, 552" verbatim.

The XI is the only Sig 556 I own, not familiar with the stock adapter for the other models. The receiver mount just has a "hook" that slots into the left side of the receiver and then a roll pin in driven in through the adapter-->receiver-->adapter on the right side of the receiver. So if the receiver on your 556 is similar it should work. I am FAR from an expert on these.

Double tap

Pics of it on the gun?

Thanks - I see how it works with the MCX/MPX stocks now.  I never had an MPX to know.

It fits the "classic" style lower receiver and permits the MPX style stocks, which is another option.  There is an MPX AR Tube adapter that is an aftermarket part made by some vendor - Its on optics planet so gives some more options.

Was busy this week and haven't decided how I want to go with the sling mount protruding. So I stuck an Allen wrench through the roll pin hole and hope to get to it this weekend.

Glad it works on the classic. Opens up some options. Wanted my 556XI to be a side folder. Might as well since it doesn't actually need the buffer tube.

does this mean its a double folder?  folds once at the original hinge and then again with the new MCX stock?

No, it hooks in solid to the left side of the receiver and then the roll pin is driven through the right side of the receiver where it looks like it would fold but does not. At least that is how it is on my 556xi PSB pistol. Then you can use the rail to put on any brace, stock (if you SBR it), or adapter that can attach to a 1913/picatinny rail.

A company used to make an adapter (DDLES) that used that pivot hinge to fold but they are out of business or otherwise don't make them anymore so I was looking for another option. I bought the XI used and thought it was a folder when I bought it based on that pivot, I was pretty disappointed when I got home and realized the brace didn't fold like on my MPX.