I would strongly suggest contacting a Chartered Accountant about that...
As far as I know the Tax Stamp is a necessary part of your business, hence a deductible expense same a property or business taxes are especially if you sell the item...
It also depends if you are a business dealing in "outdoor" stuff or just a private collector then if the first you may and the second you can't...
I can't remember how Capital Gains works in the USA but if you sell the firearm the "tax stamp" is then a loss and it would reduce the amount you would have to pay...
I started a small company dealing in outdoor equipment--clothing, knives, flashlights, optics, fishing rods & reels, fishing tackle etc. just so I could buy stuff at wholesale pricing...Took out the necessary government licenses, I file an income statement every year and I show enough sales to be a legal business but declare a loss every year what with taxes, shipping, duties, excise tax, exchange etc...