I had a go on the WA M4 the other day.
I must say the recoil is much better than I expected, truly a nice experience to shoot it. The gun doesn't feel right without a mag in, way too light. I noticed I could hear a slight twang on the recoil spring as it fired but it was of no big concern.
The owner told me he only loads 20 rounds per mag as any more (in typical Isle of Man weather) causes problems. Sadly before we could shoot it much more it broke - some problem with the hammer not locking back (has since been fixed).
Reading through (a good portion) of this thread, the gun has