Ignore the Recoil Spring Hypochondriacs. The recently imported P1 pistols came out of Bundeswehr war reserve; most are fresh from rebuild and do NOT presumptively need new springs. A pistol will tell you by feel and function if the recoil spring is weak. If it feels good, and works, don't fix it.
For those who can't --or won't--bring themselves to just leave it alone, here is the unexpurgated version:
Dismount the slide and push each recoil spring guide in turn all the way rearward. Feel for steady smooth movement. The recoil spring should not go ?solid? before the recoil spring guide contacts the rear of the frame. When fully compressed by the recoil spring guide, the recoil spring should still have a little room left between the rear coils to compress; test it by prying with your fingernail. If it doesn?t, the spring is too long; also, going completely ?solid? will prematurely fatigue a spring. Original factory springs vary considerably, but all use 0.6mm wire, coil diameter of 5.3mm, 41-1/2 to 43-1/2 coils with closed and ground ends. A new spring will measure about 123mm at rest; after a little use it will take a set to about 115-117mm. If it?s kinked or appreciably shorter than that, replace it. An excessively long, or excessively strong spring (thicker wire or more coils) may prevent full slide travel and/or shorten the cycling time, which may not get the slide back far enough to securely pick up the next round or try to strip out the round before the magazine spring has lifted the cartridge high enough to be correctly fed.
CAUTION! Great care MUST be exercised in checking or changing a recoil spring: the spring guide and spring, when compressed, must not be allowed to snap forward freely, as that WILL damage the frame by chipping or swaging out the guide retaining notch (thus allowing the spring and guide to fly out whenever the pistol is field-stripped). EASE the guide forward into its notch. Also take care not to nick or scratch the recoil springs with a tool, as scoring will lead to premature failure of the spring. A bamboo skewer sharpened to a chisel edge works well, and won?t leave marks on the spring or the frame.
If, after replacing the springs, you find that the slide will not go back on the gun, don't try to force it. It's likely that you have not correctly re-seated the recoil spring guides in the frame notches.
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