I had great difficulty getting 10 rounds in my M&P40C mags when they were new. This was before I got an Uplula loader. I kept the mags loaded fully for a week but they were still to hard to load. What I did was load & unload them a half dozen times & that helped a lot & then keep them loaded with as many as you can get in.. Didn't help the fingers any, though, lol. They were sore for a week. I believe the springs need exercising both in compression & decompression. Then I started to actually shoot the gun at the range. Even though it's no longer hard to load by hand I still use the Uplula. I'd be hesitant to make any modifications on the spring or follower. You might run into feeding problems once the springs loosened up.

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Former USN, Retired US Army