My 40c doesn't fail to feed. It just has very sluggish slide movement intermittently. It feels very odd when it happens.

I went to the range today. I had been trying to compress the mag spring for the past 5 days. It didn't work so while at the range I cut a coil off of the mag spring. I noticed an improvement, but I still had some slide sluggishness. I clipped off 1 more coil and then I didn't notice any more problem. I think the problem is that the mag spring was too strong for the recoil spring. I will try and go back to the range next weekend to do one more check. If I don't notice any problem I will believe that my fix truly worked.

I know some folks on this forum have said not to cut the mag spring, but after I did the problem went away. I tried some rapid fire and the mag spring was still plenty strong enough to support fast cycling.

I may not have mentioned previously, but the sluggish slide movement only occurred with the 10 round mag. My 40c also came with a 15 round mag which caused no issue with slide movement. I noticed that both the 10 round and 15 round mag had the exact same spring (same length and number of coils). Having the same spring in both mags, the longer 15 round mag had significantly less spring tension than the 10 rounder. The 10 round mag has the pinky extension so my grip is effectively the same with either mag. the only real difference seems to be spring tension. That is why I decided to lower the spring tension in the problem mag to more closely match the tension in the longer mag. The only really effective way to lower the tension was to cut out 2 coils. Again, it seemed to work. I definitely want one more range trip before I will be fully satisfied.