Deducting purchases for purposes of training for work

I work in a state law enforcement job that requires certification in use of a firearm, however we are not given one for training and it is expected/heavily recommended to purchase our own personal firearm for training to complete an annual recertification to maintain employment. It has been explained that I can deduct this purchase from my taxes. Is this true? If so, are there limits to this action? Is only a portion returned of the purchase amount? What is required for this?