I have read how tons of people have converted their 40c to a 9mm by buying aftermarket barrel and 9mm mags. I also got the impression that you can't convert the 9c to a 40S&W. Is this true and if so why?

I'm not 100% but I think it's because the 40 had is too big for the 9 slide

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Sorry, it won't work. The barrel hood on the .40 is about .030" larger than the 9mm. A 9mm barrel will fit into a .40 slide, but not the other way around.

Which begs the question, could you try to buy a 40C slide and a couple of mags ? Still, you'd likely be close to the cost of a new gun at that point. NEVER underestimate the NEED for another gun purchase :-)

__________________
Dave

I have both the M&Pc in 9mm and 40 SW and was wondering if the 40 top end will work on the 9mm lower (my 9mm has the thumb safety and I like that feature for AWIB carry). I looked at them and it appears to me that the ejectors are different. I stopped at that point. Maybe someone else has taken it further than I have. I'd like to know the answer too.

Out
West

The reason such a conversion is not practical is because the breech face required by the head of the cartridge is of different dimensions, with the .40 S&W having the larger dimensions. This is why it is possible to go from the larger cartridge to the smaller with maybe only a barrel change, but even that is not advisable due to differences in the breech face, which would negatively affect the grip the extractor has on the cartridge rim, and placement of the ejector, which will be in the wrong place, rendering ejection more problematic. Although people do it, it is not a good idea, and definitely such a conversion should NOT be relied on for serious work such as self-defense. The only one of these conversions that seems to work is .40 to .357 SIG, and that is because the cartridges have the same head dimensions and therefore use the same breech face. In the SIG and S&W brands, magazines are interchangeable between .40 and .357 SIG, but NOT in Glock, as Glock has found that a minor change is required in the magazines for the two calibers to get utmost reliability.

The 9/.40/.357 series uses the same frames for all 3 calibers. Only the slides and magazines are different. The extractors and ejectors are the same for all 3 calibers, as is the striker and recoil spring assembly. Due to this, many people report being able to drop a 9mm barrel in a .40/.357 Sig slide and have it run reliably.

The ejector on a pistol with a thumb safety is a different part than one without, due to the construction of the sear blocks. Their dimensions in reference to the breech face are the same, however.

You can swap the top ends between your .40 and 9mm (assuming they are both either a F/S or Compact). I have in fact done so with mine, just to verify it can be done. My 9FS comes with the thumb safety, the .40FS does not. They work just fine.

Thanks for the clear explanation on this subject. Now that I am starting to shoot more as well as reload I am interested in more calibers so I guess I need to buy another gun!