If you want a factory SIG sidfolder you will need a new lower recever. The upper is the "fireram" with the serial number, so you just need to pick up a lower from SIG USA or on Gunbroker. Some guys prefer the M4 type slider stocked lower, you might be able to just trade lowers straight across in a trade!

The other option is to buy a hinge mechanism from ACE that lets you put an ACE folder or turn an M4 tube slider stock into a side folder. In theory this is the best of both worlds as you can adjust length and fold for compactness.

If you need some help with the ACE components to pull that off, just let me know and I will post some links to what exactly you need to do it!

Sven

[ETA] You can swap out the fishgill forend for the new 551 style handguards, there are a little lighter and look a bit nicer in my opinion. They are available at SIG USA for $99 IIRC, or you can usually find a used set on gunbroker for about $50.

I would also recommend registering over at sigarms556.com, all the bits you are looking for are often available on their own SIG 556 specific EE.

[ETA 2] you need a SIG specific rear sight, they are much lower. Botach had the best price on them if you want to risk ordering from them (I believe the brand is ITAC] for about $99. other option is to just go with a set of midwest industries front and rear sights, a little more expensive but nice.

I ordered an ACE folder kit today for my rifle, and posted a WTB in the EE for a Sig 551 hand guard set.... If anyone knows of a good place to pickup the 551 forend for a reasonable price, please point me in the right direction.

Thanks in advance.
-Tac