Bar-Sto says on a .40 to 9mm conversion for P226/9:That's the official line they have in their FAQ's section. Anyone ever actually done it without a slide conversion kit?For Sig firearms the 229,226. You only need to change the barrel and use 9mm magazines. On the 239 you only change the barrel and can use the 40 but not the 357 sig mags. On all these you can get factory 9mm mags for those.
No spring changes needed. The breech face is fine and the extractor on these type of firearms springs over far enough to catch the case head and extract reliably.