Appreciating the responses. Based on them, I think I'd be most comfortable with a .357 Sig barrel in a .40 P229 or a Glock 23, since the gun is basically the same with a slightly narrower barrel (inside, same exterior dimensions). The .40 case being necked down to .357 would let the magazines, extractor, etc. all function the same.
Beretta PX4 .40 S&W - Beretta 92FS 9x19 Para - Rock Island Armory GI 1911 .45 ACP - Colt 1968 Detective Special .38 Spc. - Ruger LCP .380 ACP - Daniel Defense M4V1 Carbine 5.56x45 - Alpine Industries M1 Carbine .30 - Steyr-Mannlicher M1895 8x56 - Ruger 10/22 .22LR - Remington 870 Express Tactical 12GA - 1943 Izhevsk Mosin-Nagant M91/30 7.62x54R