Why must I change magazines if I convert my Sig P239?

[Airman193SOS]

My Sig P239 is chambered for 357SIG. While I don't have any intention of converting it to .40 S&W via drop-in barrel (I've got that caliber covered with my Glock 27), I do wonder why the magazine is not round-interchangeable. I can't think of any other handgun so chambered that requires new magazines for the transition. Does anybody have any idea why this particular gun requires it?

 

[Claude Clay]

my p239 was born as a 40. i have a 357 conv bbl and a 9mm conv bbl. all 5 of my 40 mags function perfect with all 3 bbls.. these are all marked 40 mags. those marked 357 only seem to work w/ 357. go figure

 

[Phil DeGraves]

It doesn't require it. 40S&W and 357SIG mags are identical. Whoever is telling you that just wants to sell you more magazines. (More magazines is not a bad thing though...)

 

[DWARREN123]

Glock marks their mags as either 357 SIG or 49 S&W, they are the same. Should be the same for your SIG.
The 357 SIG is a necked down 40 S&W, basically.

 

[Josh Aston]

49 S&W

Where can I get one of these .49s?

 

[Airman193SOS]

The 357 SIG is a necked down 40 S&W, basically.

I know that.

Whoever is telling you that just wants to sell you more magazines.

I would imagine that Sig does want to sell more magazines, because they're the ones that require it. See page 24 of the manual. The Sig P229, shown right above the P239 in the manual, requires no such magazine swap.

So, I'll reiterate: why do they require different magazines?

 

[AK103K]

The early guns had caliber specific mags. My P239 in 357SIG came with 357SIG only mags. They have a recess in the mag where the "shoulder" rides. A .40 case will not fit in the mag. The 357SIG's will work fine in the old .40 marked mags.

All the new mags I've seen and bought are marked .40/357SIG and will take both.