Purchased a used KWA kmp9 that works but had broken/missing cylinder return springs (I noticed that the cylinder nozzle would just flop around even though the seller stated that the cylinder springs (among other parts) were replaced. FPS was around 400 with 0.20g and tested with xcortech so seals appeared to be ok. I read that in addition to the cylinder return springs, part #87, other common parts to fail included the disconnector part #153 (breaks with wear), hammer part #26 (axle posts break) and breech base part #19 (pins that attach to the cylinder return springs break off) so I ordered and received the hardened steel parts from Wii Tech to improve durability since I had the kmp9 disassembled anyway (which is how I noted that the cylinder returns springs were actually missing and not replaced) and I did not want to have trouble getting replacement parts in the future if the kmp9 was discontinued (I had this problem getting parts in the past from other discontinued products). I also ordered three extra piston seals that fit only on the Wii Tech "loading nozzle chassis" (Wii Tech's name for the "breech base") as they are cheap and I did not want to have future supply problems if the seals eventually wore out. My problem is that when I placed the Wii Tech loading nozzle chassis into the KWA stock polymer cylinder, the seal was tight enough that the nozzle springs would not return the cylinder back to the breach on their own. I wanted to use the stock polymer nozzle as it is readily available (it is KWA NS2/KSC system 7) and felt the polymer would have "kinder" wear characteristics against the bolt carrier compared to the aluminum Wii Tech cylinder. On the Wii Tech site, the cnc loading nozzle chassis has a product number of "4020". This part "4020" is for the KSC system 7 which should be the same as the KWA NS2 (I own a number of KSC system 7 products and all the parts have been compatible with the KWA NS2 replacement parts). Part #4020 is the only replacement part for the breech base listed on their site and it differs from the original breech base in that it does not have a separate screw-in type main seal cap and seal (KWA parts #127, #129 and #133) but is a one piece unit (incorporating a steel pin for the cylinder return springs), which is why I also purchased three extra main seals just in case it became difficult to get Wii Tech parts in the future. When I did some extensive searching on the net, I did notice that some Chinese sites had an "older" version of the Wii Tech loading nozzle chassis that did apear to have a separate screwed in main seal cap and seal (the main seal cap appeared to be anodized a gold color and was included and pre-assembled on the loading nozzle chassis with a Phillips head screw) just like the oem KSC/KWA breech base. I do not recall the part number for this "older" part but it was different from "4020". I also noticed that the "4020" part was listed as "new" which leads me to believe that Wii Tech came out with this one piece loading nozzle chassis to cut down on machining costs or to prevent the main cap seal from loosening from the breech base. After spending $30 plus shipping on the "new" version Wii Tech part, I did not want to risk buying the "older" version part from a company I have never ordered from (particularly from China), just to have them send me another of the one piece "newer" part due to availability. At any rate, due to the increased seal of the part "4020", I have not tested it as my kmp9 already shoots at 400fps which is the field limit at my field and I would guess that with a even greater seal, this would up the fps beyond field limits. In addition, more importantly, I did not know if this would cause any feeding problems and possibly jam or damage the gun (particularly in auto) if the cylinder did not fully return to the breech base given the added friction of the seal. Unfortunately, this means that my kmp9 is still suceptible to the pin breakage on the breech base given the poor quality of the cast metal used by KWA as the oem pins are cast as one piece as part of the breech base unlike the steel pin in the Wii Tech product. Does anyone know if the blow back pressue is enough to force the cylinder back into the breech base on its own (I have heard that some people simply do not use the cylinder springs anyway) or will this cause damage and feeding issues if the cylinder either does not fully retract into the breech base or the timing is adversely affected? Thank you so much for any input you may have. I really appreciate your help on this.