This conversion barrel is specific to the newest XD; the XDm. What is a conversion barrel? A conversion barrel allows you to convert your large caliber XD (.40SW) down to a smaller caliber (357SIG) for the benefit of practice and plinking.
Just a description that made me laugh. Anyone who has shot both calibers knows exactly what I'm talking about. I'm not aware of anyone who trains with a 357 for their defensive 40...plenty go the other way around, however.
Back to the OP-- 357sig is a fun round, and the conversion should be a piece of cake. But only invest in that barrel if you'd prefer
the 357sig to the 40sw. I could understand wanting the 357 for defense, but practice/plinking with a 40 is just a whole lot less expensive.