For $400, it is not worth depreciation as you will get $80 per year according to IRS standards.
However, depreciation is considered "long-term". Let's suppose a rifle is only worth $200 and you put in $2,300 to repair and refinish the item AND its only purpose is to be a show piece of your skills and expertise. Then, you depreciate the item as you spent more than 50% of the worth and it becomes a new item. Plus, since you intend on showing it over time, it needs to be depreciated.
On another example, if you have a rifle that is only worth $200 and you spend $200 on a new stock. The item is still a show piece item and you spent more than 50% of the cost to repair & refinish the item. Then (technically) you still need to depreciate it. However, nobody is gong to split hair if you decide to expense it since your depreciation is only $80 per year.
In short, it's up to the accountant's professional judgment on a lot of these particular items. I hope this helps!
PS It would fall into the depreciation of "Props and Settings"