I know(at least I think I know) the Sig 556 pistol has a cap that can be removed and a standard AR buffer tube and stock can be installed.  I really would love to make an SBR Sig 556 swat. But since I have my AR SBR and love it, it is really hard to justify the $2000 I would wind up spending on something with a main purpose of "range toy".

Then I saw the Sig 522 Swat pistol and [idea]

Sig 522 swat SBR.  That would be a lot of fun!

So can the cap be removes on a 522 pistol? Ar there threads for an AR buffer tube?

On the 556 pistol they are made with the early version lower that takes a buffer tube type AR stock.

On the 522 pistol they never had a lower that took an AR style stock, so the end cap is completely different. If removed, the end cap can be replaced with a stock from a 556 classic or Swiss stock from a 55X series rifle. Given of course that it has the tax stamp.

If you want to put a 556 classic or swiss stock on a 556 pistol then you would have to swap out the lower for a classic lower.

DDLES makes a conversion hinge, super clean.  Here is a thread on the 556 forum about it.

http://sigarms556.com/viewtopic.php?t=9119&sid=aa4a60f41f3de58cf178d36e76518a1e

That is most excellent!!!  Thanks!!!

Does anyone know of a source for the stock release button/catch and associated springs to make the DDLES adapter work on a P522?  The stock adapter does not appear to come with them and I don't have an existing stock to take them from with my P522!

Nobody that I know of sells the hardware for a side folding stock. Your best bet would probably be finding someone in the EE or on sigarms556.com classifieds selling a classic stock and cannibalizing the parts from it. Probably could pick up on of those stocks for around 75-100 bucks.