How to build a .22 LR Conversion Slide/Barrel for a P239 .357 SIG?

A couple of thoughts.

Don't know if the 'guts' of the 229 slide and 239 mate up, so I'd look there first - as in firing pin block lifter, etc. on your conversion slide.

Breech face, extractor, etc is next place to look. Perhaps there's a SIG 22 barrel that does fit.

I don't have a 239, so I can't help much with photos, etc.

In the FWIW commentary section (smile):

With the P250, the 'swiss army knife' of the SIG line, the 22 slide is aluminum, and the recoil spring assembly fairly weak all things considered. Not necessarily applicable but just for info, the 22 and 380 P250 shared odd sized compact and subcompact modules, and both have shorter slides. 380 barrel is conventional pattern but highly relieved; 22 barrel actually lies closer to the hand. I offer this observation just so you can see it is quite possible that different slides might work on the same receiver... you'll have to check measurements.

Note that SIG chose NOT to create a subcaliber device for the 320 by producing the 322. I have ZERO idea about any interchangeability that it might have.

So as another response noted, SIGs own choice not to make a 239 22 unit is pretty indicative of difficulty vs potential market. But don't let that deter you if you're really interested in trying.

I can't think of any hobby gunsmithing books that would even provide an insight, but if you're a potentially serious pistolsmith, let me recommend Major George Nonte's book "Pistolsmithing". Maybe there's some techniques in there that might help you design / define your project.

Strikes me you could also contact one or more of the folks that make 22 conversions aftermarket and see if they can offer any insights.

Hope this was helpful - keep us all informed on your progress, if you would?

Nothing to feed it with, ala "Magazine" as a start...

While you can see here, the Takedown Lever "cut" (#2) while close to being correct, the Disconnector "cut" (#1) is off a bit, but even without a barrel assembly, the P229 Slides chin too "low" and impacts the frame's dust cover...



Likely one reason a rimfire conversion was never considered, is due to the P239 being designed as a "Back-Up" for Law Enforcement, in the same caliber they carried with their duty weapon. Rimfire "conversions" were a later introduction for low cost training with "primary" weapons.