Actually according to Snopes, listing guns on your Tax Return is FALSE.
http://www.snopes.com/politics/guns/taxreturns.asp
What you linked to and is true is the Blair Holt Firearm Licensing bill... this is very different than the tax return thing you posted... also at towards the bottom of the Snope's Blair Holt link you provided you'll see that the Blair Holt bill has really no chance of passing... Oh... and read the Snope's entry on the Blair Holt Bill closely and you'll see there are a number of other falsehoods being circulated about this Bill...