I have a Sig Sauer P239 in .40 S&W that only had 50 rounds through it when I bought it.

Three times in about 500 rounds shooting it I have seen it eject the last live round of the magazine out the ejection port instead of feeding it into the chamber. So I fire six shots, and then I pick up a live round off the ground in front of the shooting bench.

The first two times I saw it happen I was using the Mec-Gar magazines it came with. I ordered 5 new magazines (also Mec-Gar) and installed a fresh Sig recoil spring but it happened again.

I hear that there are also magazines made by Sig Sauer themselves and not by Mec-Gar. I was thinking about trying to find some but they are $41 on MidwayUSA and I already have 7 of the Mec-Gars.

Could it be related to riding the slide stop?

What do you think I can do to fix it?