Well, you're in for a surprise. You can definitely drop in a factory 357 Sig barrel in your 2340. No problem. BUT, it's going to set you back $150 minimum for a new one. You might be able to pick up a used one in the classifieds here: http://www.sigforum.com or look on ebay.I figure they cannot be more than $50-$60, if that...
The 357 Sig is a great round. I have several pistols chambered for it and prefer it over the .40.