I just picked up a 229 in .40, but I want to shoot 9mm also. Will any Sig 229 barrel fit or is the hole in the end of the slide larger, like in Glocks, where a conversion barrel is required?
Don't know but I ordered a .357sig. conversion barrel myself. With .357sig the mags are interchangeable. For the 9mm you'd probably need a different mag.
I just put your question on Sigforum and this was the first reply.
Your friend needs to buy a conversion barrel. On the P229 the locking inserts are different on the 9mm and 357/40 guns. 9mm ammunition will feed from the 40 magazines, although 9mm P229-1 magazines may be better. It is also possible he will need a 9mm recoil spring.