Also, given that he didn't pay for the guns, the entire purchase price (less expenses related to the sale) is profit. Profit is defined as sale price - purchase price - expenses. Since the purchase price is zero, profit in his case would be sale price - 0 - expenses.
It isn't profit. It is an inheritance and therefore not a profit at all. If you inherit a car and then later sell it you don't pay taxes on selling the car under Federal law.
The only primary exceptions to the above would be if you sold them for more than they were worth at fair market value or waited several years and they appreciated during that time. If that happened then you would owe capital gains taxes on the increase in value of the items from the date that you inherited.