I own an HK USP Compact .357 Sig that I bought back in 2001.  I am wanting to exchange the barrel for a .40 cal and have heard that it is as simple as just dropping in the new barrel.  When I asked H&K they told me that at this time they do not offer a conversion kit for the USP Compact.  So what's the story?  I've gotten two different versions and unsure if I should just try to drop in a new barrel and give it a try without blowing myself or my gun up.

I would think calling Jarvis Barrels up would get a replacement bbl in 40 SW.

On Sigs, the conversion difference is barrel & recoil spring. Since you already have the stouter spring on the .357, a .40 barrel should be all you need? Mags, etc. all remain the same.

Try Nowlin & Bar-Sto for barrel options as well.

Good luck!

HK does not sell a conversion "kit" for the .357 to .40 any more than they sell a "kit" for the .40 to .357.  Just get a HK .40 barrel from Marc at TROS or from Cal's and drop it in.  It will work just fine.  I have heard nothing about needing to do any kind of spring swapping regardless of which way you go.  In my .40 that converted to .357 nothing was needed but the barrel and several I know going the other way did no other changes either.

Quoted: I have heard nothing about needing to do any kind of spring swapping regardless of which way you go.

Sig does it on their P-239 guns, could be due to the slide weight unique to Sigs?