I just installed a flat faced trigger and fss with the silver spring (supposedly the lightest) on my pro core 5" today and I am getting around 5lb 5-10oz pulls. This is almost heavier than the stock pro series trigger. Any ideas why this may be happening?

I did not adjust the trigger bar, as the pistol functioned fine without adjusting. Shot a little over 100 rds through the gun and came away very disappointed.

re-install the factory return spring and it will be lighter.

But shouldn't the pull still be around 4-4.5lbs rather than close to 6lbs with the lighter Apex spring?

Factory return trigger is lighter than the apex spring, did you polished any of the parts? that will smoother the mechanism even more.

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M&P 9, Shield9.

Yup. And the striker block polishing or replacement is critical to improvement. The channel (under the rear sight) may need to be smoothed in order for the block to move smoothly. I got my Shield down to under 5 lbs, Apex sear and polishing only. Stock trigger.

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CERAKOTE Svcs.
Class 01 FFL