I haven't had time to read through all of this thread, so please excuse me if I'm wasting bandwidth and repeating something already said ...
The fiber insert in the trigger spring is intended to dampen harmonics experienced by the trigger spring during recoil. The spring will last significantly longer with the fiber insert inside the spring.
Way back when I attended a Sigma Series armorer class this was briefly discussed. We were told that the harmonic forces experienced by the small spring could weaken and eventually cause failure of the spring, although the use of the fiber insert helped mitigate the potential for this to occur and extended the service life of the spring.
We were warned that if the pistol were submerged or otherwise treated so that the fiber insert became soaked and liquid-laden that it might slip out of the spring at some point. In that case the spring would have to be replaced with a new one (which came with the fiber insert already installed). One of the instructors did mention that if this were to occur in field conditions where access to a new spring & fiber insert were delayed that wadding up a the filter from a cigarette would probably allow for use until proper maintenance and parts replacement was possible.
The M&P trigger spring is of the same design (since it has proven itself in extended testing in the Sigma series and during R&D of the M&P pistols series).
The last spare trigger springs I received for a M&P parts order came with the fiber inserts installed in the coil spring.
FWIW, in the M&P armorer class we were told that it's recommended to replace the trigger spring (and striker spring) every 5,000 rounds.
Inserting the trigger pin through the front closed coil of the spring can be a bit tricky. Armorers can use a 'slave pin' of a type originally used in the Sigma Series armorer class, or a pin punch and a third hand.
The open spring coil end is located toward the rear and the 'open part' of the coil faces upward when installed in the trigger bar.
Personally, I avoid introducing excessive solvent/CLP to the area of the trigger spring when cleaning my M&P's to avoid having the fiber insert become needlessly soaked.
A bit of trivia which may, or may not, be interesting to anyone is that in my last Glock armorer class it was implied that it might be prudent to start replacing the trigger spring periodically. Armorers also received a written update that they should inspect the trigger springs in their in-service Glocks.
The revised manual page insert said to check for:
Broken or missing springs.
Springs with coils that are bent, crimped or stretched.
Hooks on springs which show any signs of stress marks.
Springs which are not the current version (grey finish).
Springs which are not installed in the proper "S" configuration.
Back to the Sigma (and M&P) trigger spring & insert ...
I was initially a bit taken aback by the 'low-tech' spring fiber insert when I first saw it in the Sigma Series armorer class. Then in my first Colt AR armorer class I was told that the small plastic insert in the AR extractor spring served a similar purpose of dampening harmonics ... and saw how the plastic part became distorted and mangled.
The use of a fiber insert which wasn't easily mangled, and which actually increased the potential service life of a spring in a working gun, didn't seem like such a bad idea after that ...
It might also be helpful to consider that the trigger spring in the Sigma & M&P are located closer to the 'trigger end' of the gun, which is where it might be possible for more recoil forces to be transmitted to the spring ... and then the idea of a dampening insert seems like a good idea.
There are always compromises and tradeoffs with different firearm designs.
The trigger spring of the M&P and Sigma is easier to install and remove than that used in the 99 series, for example. Installing the open coil at the 'front end' of the trigger spring in the sear housing block's hole is a bit less easy.
Just don't submerge a Sigma or M&P pistol without making sure the insert doesn't slip out of the spring, and give it a chance to dry out, if possible. If the insert slips out and the spring might not last as long as it would have with the insert present to dampen vibration and mitigate recoil harmonics.
The fiber insert in the trigger spring is intended to dampen harmonics experienced by the trigger spring during recoil. The spring will last significantly longer with the fiber insert inside the spring.
Way back when I attended a Sigma Series armorer class this was briefly discussed. We were told that the harmonic forces experienced by the small spring could weaken and eventually cause failure of the spring, although the use of the fiber insert helped mitigate the potential for this to occur and extended the service life of the spring.
We were warned that if the pistol were submerged or otherwise treated so that the fiber insert became soaked and liquid-laden that it might slip out of the spring at some point. In that case the spring would have to be replaced with a new one (which came with the fiber insert already installed). One of the instructors did mention that if this were to occur in field conditions where access to a new spring & fiber insert were delayed that wadding up a the filter from a cigarette would probably allow for use until proper maintenance and parts replacement was possible.
The M&P trigger spring is of the same design (since it has proven itself in extended testing in the Sigma series and during R&D of the M&P pistols series).
The last spare trigger springs I received for a M&P parts order came with the fiber inserts installed in the coil spring.
FWIW, in the M&P armorer class we were told that it's recommended to replace the trigger spring (and striker spring) every 5,000 rounds.
Inserting the trigger pin through the front closed coil of the spring can be a bit tricky. Armorers can use a 'slave pin' of a type originally used in the Sigma Series armorer class, or a pin punch and a third hand.

The open spring coil end is located toward the rear and the 'open part' of the coil faces upward when installed in the trigger bar.
Personally, I avoid introducing excessive solvent/CLP to the area of the trigger spring when cleaning my M&P's to avoid having the fiber insert become needlessly soaked.
A bit of trivia which may, or may not, be interesting to anyone is that in my last Glock armorer class it was implied that it might be prudent to start replacing the trigger spring periodically. Armorers also received a written update that they should inspect the trigger springs in their in-service Glocks.
The revised manual page insert said to check for:
Broken or missing springs.
Springs with coils that are bent, crimped or stretched.
Hooks on springs which show any signs of stress marks.
Springs which are not the current version (grey finish).
Springs which are not installed in the proper "S" configuration.
Back to the Sigma (and M&P) trigger spring & insert ...
I was initially a bit taken aback by the 'low-tech' spring fiber insert when I first saw it in the Sigma Series armorer class. Then in my first Colt AR armorer class I was told that the small plastic insert in the AR extractor spring served a similar purpose of dampening harmonics ... and saw how the plastic part became distorted and mangled.
The use of a fiber insert which wasn't easily mangled, and which actually increased the potential service life of a spring in a working gun, didn't seem like such a bad idea after that ...
It might also be helpful to consider that the trigger spring in the Sigma & M&P are located closer to the 'trigger end' of the gun, which is where it might be possible for more recoil forces to be transmitted to the spring ... and then the idea of a dampening insert seems like a good idea.
There are always compromises and tradeoffs with different firearm designs.
The trigger spring of the M&P and Sigma is easier to install and remove than that used in the 99 series, for example. Installing the open coil at the 'front end' of the trigger spring in the sear housing block's hole is a bit less easy.
Just don't submerge a Sigma or M&P pistol without making sure the insert doesn't slip out of the spring, and give it a chance to dry out, if possible. If the insert slips out and the spring might not last as long as it would have with the insert present to dampen vibration and mitigate recoil harmonics.
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