"To be deductible, a business expense must be both ordinary and necessary. An ordinary expense is one that is common and accepted in your trade or business. A necessary expense is one that is helpful and appropriate for your trade or business. An expense does not have to be indispensable to be considered necessary."
For items that are mixed personal and business use, you must assign a percentage of business use and, if audited, you must be able to show some kind of reasonable calculation or method that you used to estimate the work percentage. For items that may be seen by an auditor as purely personal in nature, you may also be asked to show that you would not have purchased the item "but for" your business needs. In other words, if you are a gun collector and have a large collection of personal guns, you may be asked to show that a gun listed as a business purchase was really purchased for business and not as part of your collection (that you are trying to get a write-off for).
Also be aware that if you deduct the cost of the guns as a business expense (either by expensing them or depreciating them) their cost basis become zero. That means that if you later sell them, you have to report the sale of business property and pay income tax on the proceeds. (Likewise, if they are lost or stolen and you get an insurance payout, that money will be taxable income.)
