I want to know if anyone has put an Sig 550 lower on their Sig 556 upper?

Yes.

This is an early SIGArms 556 upper receiver( number 1577) on a SAN factory semi auto lower.
At the time I undertook this project, I had to have the factory SAN semi lower imported. This was before JDI imported them and before West Coast Armory offered their product.
It took over two years between ordering the lower to final completion.
Nowadays, you can find a SIG 550P lower easily.

Thank you what does west coast armory offer?

West Coast Armory is a gun store, gunsmith, and shooting range in Bellvue, WA. that sold SIG 556 upper receivers mated to SIG (SAN) 550P or PE90 factory semi auto lower receivers years back. I can't give you a date or timeline. They still sell 550P lowers and separate 550P uppers on Gunbroker auctions, but I don't think they sell the 556 uppers and 550P lowers as complete rifles anymore. I don't think that SIG still makes 556 rifles.

Do a search from this forum using the terms west coast armory and you should find some
threads on the topic.

I did some checking on Gunbroker. It's not West Coast Armory selling on Gunbroker, it's CO Gun Sales.
At any rate, West Coast Armory hasn't sold these hybrids for years. They do come up for sale occasionally

I purchased one of the West Coast Armory rifles about 15 years ago. Paid around
$4000 for it. Removed the top rail, front sight and purchased from Colorado Gun Sales the Swiss railed tritium diopter sight and tritium front sight. Paid $800 for this set up. Still sits unfired and not worth what I have in it, ha.

If I remember correctly, according to the WCA story, back around 2008 SIG had 250 551 rifles stuck in customs for some reason. WCA purchased the rifles and sent them to SIG USA who changed lower trigger group to semi auto and then SIG USA built uppers and added serial numbers to the upper to match the Swiss 551 lowers.

Also around that time I purchased a 556 and a de mil Swiss 551. Took the internals from the 556 lower (trigger, hammer, safety bar, etc) and put them in the 551 lower. Took the 556 upper, removed the top rail, filled in the holes and welded on the 551 Swiss dipper sight. 556 upper mated perfectly with the Swiss 551 lower and works great.

I have a 551 clone made from a 556 upper. I was fortunate enough to get Estes Adams to change the barrel to the proper Swiss one before he went to the great schutzenfest in the sky. You will need to make sure that your lower is semi-auto.

quote:

make sure that your lower is semi-auto

A select-fire lower can be configured to work fine as well, and it's not a permanent modification. You can order a semi-auto sear from gunfactory, swap it in, and remove the now-useless select-fire parts.

Wil is correct. I didn't go that route for my SAN lower.
That could be why I had to wait 2 years until Neuhausen decided to make some more semi auto lowers