If you want to do a weapon in XSI, have a look at these tutorials:
Colt 1911 (I'm currently following this tutorial to learn weaponmodeling)
And if you want to animate it later, you could have a look at
BJ's animation tutorialBut first of all.
If you want XSI (which is free and good)
You should go
HERE and download Mod tool V4.2.
And then download the Valve source add-on.
But remember that you have to register your XSI to be able to save!