I have 300 rounds through my GEN4 G21 - zero problems - I cleaned it yesterday - and when I went to load a round into the chamber the slide locked back - So I paid a little extra attention to what I was doing - and figured out - when I rack the slide on the G21 I WAS TOUCHING THE SLIDE STOP WITH MY THUMB! I also tested my G19 and my thumb does not hit the slide stop when I rack the slide. Think it is just the slight size difference + the G21 is HARDER to rack. I compared my G21 & G19 - noticed that the slide stop on my G21 has much less "spring" to it - G21 takes almost no force to get it to move up So - How much spring force should the slide stop have? I found the part on Midway it is only $6.49 I would just replace it since it is bugging me - but if the new part is the same (nothing really wrong with mine) then it would be stupid to replace it. BTW - why does Glock have a part number STAMPED on the part - but use a DIFFERENT number The slide stop is stamped 5427 which is the same as the picture on Midway But Glock part number is SP05423.