I've been having problems with my P40 feeding reliably while using the slide lock lever to release the slide when loading the first round of a magazine and also when shooting, it would fail to return to battery.
My solution? Use Hoppe's Elite Gun Oil in the 2oz squeeze bottle item # G02
I was using Rem oil in the 2 oz squeeze bottle but I had found that it would pretty much evaporate and leave the invisible Teflon coating only. It seems that the Teflon wasn't enough lubrication. I had issues with chambering bullets with more grains than 165. Even the 165s were jamming. I didn't have this problem when I had first gotten my used P40. I've upgraded the guide rod to the stainless steel part from Kahr and replaced the recoil spring months ago as preventative maintenance and for more rigidity(guide rod).
So far so good. I haven't been to the range yet but racking the slide with the slide stop lever and sling shooting has been 100% reliable so far. I had problems prior to using the different lube 75-90% of the time when using the 180 grain full metal jacket rounds. I was trying to chamber Federal Champion and Remington UMC Leadless rounds in 180 grain.
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I'm glad I didn't trade it in due to frustration. I'm very happy with my P40 now