I had issues using the FFT Wolff Spring in my 7 round mag tube, even after polishing the tube it still would not launch shells up the ramp and was causing the shells to not exit the tube fast enough and up the carrier far enough that the carrier would hang up. It was frustrating as hell, wasted a bunch of shells trying to trouble shoot the issue. I switched to a Carrier Comp Spring and that completely fixed the issue. Hundreds of shells later and my M4 is running like well.... an Italian made Shotgun.
Also of note was that the Carrier Comp Spring is a good bit longer than the factory spring as well as the FFT Spring. The only way I can get 7 rounds in my 7 round tube is to use true 59mm (2-3/4") shells. Most overseas stuff is too long so you will have to settle for 6 in the tube. But I talked to Marcy over at Carrier Comp and they mentioned that you can trim up to 5 coils off their spring and still have a spring that is superior to the factory spring. So just keep that in mind, I ended up leaving mine stock and just noted that if I shoot Federal 00 2-3/4" shells than I can get 7 in the tube, if I run others like Fiocchi, Wolf, etc... then I can get 6.
But in my opinion the Carrier Comp Spring is superior to the factory and to the FFT Spring that they have Wolff create for them.