[ButThreeLeftsDo]

$500 gun includes the current 10% tax. A 20% rise to 30% would put the gun at $600.

[FtrPilot]

$500 gun includes the current 10% tax.

Correct. Therefore, the price of the gun prior to tax is $454.55.

30% of $454.55 = 136.37

New price would be $590.92