Hoping someone will have some insight, and wondering if anyone subscribes to my theory--

I've had my SP2340 for only around 2 years+, bought it in .40 and it's always performed as you'd expect from Sig. I picked up a .357Sig drop-in barrel about six months ago. I shot crappy when I first got the barrel, figured just a matter of getting used to a new cartridge.

Recently I took it to the range to concentrate on it, as I hadn't been shooting it much among my guns(usually shot the gun in .40). I was shooting way low. To hit in the inner black I had to line up the sights with the very top sliver of the circle barely visible(large 50 yard target). I had one of the guys at the shop look at the sights after 100 rounds of this. Sights hadn't been knocked out of alignment.
Tried another box, same results.

Shooting in .40 has always been accurate. So here's my theory: After stripping it to clean and inspect it at home, I had both barrels side by side and noticed the hole where the notch of the back end of the guide rod fits in were different sizes-357 larger. There was no play in the 357 barrel, but the spring could account for that. Because the sights are fine, the .40 shoots fine, and there have been no other issues, this is the only trouble shooting item I could come up with.

Long winded, I know

Most likely the difference in velocity between 40S&W and 357SiG. Since 357SiG is a faster bullet than the 40S&W it will leave the barrel earlier in the flip, and so will shoot low. Perfectly normal and not to worry.

What sig_230 said.

To really "dial it in" you will need to peplace your front sight with a lower one...but then the .40's won't shoot to POA, so decide which you want to be "centered" and go with that.

I will add that if this is something you consider to be a serious issue, you may be able to play with various loads and sight heights and find a compromise with both calibers. My G31 is my dedicated .357Sig and the .40 barrel is just for grins...and in case I can't find .357Sig ammo, etc. Obviously, you could also have some sort of adjustable rear added...but this sort of defeats the purpose of a service pistol...JMO

Thanks for the replies, makes perfect sense. Of course I'll point this out to the guy who checked my sights at the range, rib him a bit since we're on friendly terms. Especially because I bought the gun from them. To his credit he was pretty busy and is generally the shotgun master of the shop.