On the 9 mm sigma theres a spring looped on both ends with the felt down the middle located just above the trigger that returns the trigger. Is this easily removed? Is it riveted? Can you just take the slide off and use a pick to remove it and replace it?

Don't touch it. w/o that felt the trigger spring will break.

The reason I asked is because I have a replacement. Does it just come right out?

The trigger return spring has one end hooked through a hole in the trigger draw bar, and the other end of the spring is held in place by the crosspin through the frame that holds the trigger assembly in place. It is not particularly difficult for an armourer to detail strip a Sigma, but it is easy to mess up the trigger assembly if you don't know what you are doing.
http://www.youtube.com/watch?v=atOw-...ature=youtu.be
Shows how to detail strip. Note: This video is for the similar SD. There presently is not a kit for the Sigma.

Why are you wanting to replace the Sigma trigger return spring?

It is not the main factor in the Sigma trigger pull weight (that's the sear springs) so putting in a lighter trigger spring is practically pointless (maybe 1# less).

It is not intended to be field serviceable by the user.
I can detail strip a Sigma, but if a trigger spring ever breaks in one of mine, it is going back to S&W on their dime. They'll probably replace the entire trigger assembly.

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Science plus Art

The reason for replacing it is getting the 1# less you mentioned. Thanks for the answer by the way best one yet. Another questions for you. What's the reference I hear to a "Free State return spring?" on different forums. Also do the SD's, M&P's and the sigma all use a trigger return spring with the felt inside?