So I had my Sigma 9 pulled down to clean and polish the sear, I noticed the trigger return spring has a felt material inside of it, what is it's purpose? Make the spring tighter when stretched?

it is there to protect the spring from breakage.

How does that keep the spring from breaking?

I think it is there to limit and dampen vibrations. Other posts reported that S&W states that this material is a necessary part.

I read in one of the Sigma trigger threads that some people were removing it to lighten the trigger pull and thought I would give it a try. I don't want to do anything drastic, I've polished the sear assembly and it made a drastic improvment in the trigger.

Since you've polished the sear, that's about as much smooth as you're going to get.

I suggest not removing any springs, since most of the 'weight' you feel is from the striker spring, and you're not going to lighten that one up. My $0.02 - removing spring(s) won't make it feel lighter, but might make it less reliable, so I wouldn't do it...

I agree -- I would leave it as is. Which is pretty good for self-defense work and adequate for plinking.

If nothing else, think of the liability or responsibility involved in altering the gun and then someday having it get into someone else's hands -- without knowing about a spring modification.

If you want a smoother, lighter trigger, think about getting a SD, or better yet a M&P with the Apex kit installed. Or even leave striker-fired guns behind, and go with a gun with a fine single-action trigger -- a good 1911 or a Sig. The Sigma is great at what it is, which is not a smooth-triggered target pistol.

The cotton reportedly dampens vibrations that apparently caused the springs to snap in the early days of the M&Ps. Seems strange -- it is the one part of the M&P that is not top class (the same part is used in the Sigma and the SD, apparently). I have a couple of them that are more or less saturated with gun oil now -- but I would not remove the cotton, and try to keep it dry.

I would suggest you not mess with it. It's a little trickier than it looks. I did it once. Don't feel that good or that lucky to do it again. Not worth it. You have already polished the sear with noticeable benefit. If you still feel grittiness, suggest you call customer service and tell them you'd like to send it in and have them work on the grittiness. That's what they did on mine and it's amazing.

Good luck,

Icemn

Someone posted on here they sent their's bacl due to gritty trigger- might could find that post, might help.

First, This pistol is for nothing but range and open property plinking.
I have a Kimber Super Carry Pro 45acp, M&P 9mm,several Colt revolvers, Sig P238 and numerous others, so I am just trying to improve the Sigma for fun & self satisfaction. There is a night & day difference in my Sigma since I polished the sear block, plastic sear housing and I did remove the felt from the trigger return spring which does lighten the trigger pull a small amount but noticeable. I will order a spare spring just in case it causes an issue, worst that can happen is the trigger won't return to fire position if it breaks. I won't be messing with any of the other springs.

I know this is old but did you ever remove the spring? Can I ask how you did it because I can't find any info on how to remove the trigger return springs

Don't mess with the trigger return spring--it's the sear springs that people have been experimenting with.
All that happens when you remove the trigger return spring is that you have to push the trigger forward after each shot.
Original poster misunderstood spring names.

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Science plus Art

If you really want to replace it watch this video. Link

polishing the bottom of the slide where the trigger arm travels will smoothen the pull.
polishing the trigger arm where it contacts the frame and sear housing will also smooth out the trigger pull.

Honestly the best thing for a tought trigger pull is use. Go to the range fire 500 rounds and clean it. You'll be surprised next time you pick it up how much better it is !