There may be many reasons for a test fire on a firearm. The first being the customer claims something is not working correctly with their firearm. In this case you may need to test fire to see the actual malfunction they were talking about in order to better understand the issue and find the best way to resolve it. In other cases, a test fire may be part of ensuring a firearm you built yourself actually functions. With an FFL that allows the manufacturing of firearms you can purchase all the parts you want for a build and assemble the firearm from scratch. This, however, means that you should test fire before selling the firearm to ensure the product being put out for sale is a functioning one.
As someone who has spent a number of years working for live fire facilities, I have seen countless firearms I was unfamiliar with and was then asked to clean, diagnose, or fire them. This can be a little unsettling at times especially when it is a much older firearm worth a fair bit of money. In such cases I would usually admit to the customer that I am new to this make or model and that it would take me some time to do the research to complete the task the correct way. Doing research is one of the best ways to make an unfamiliar firearm safe to test fire. The firearm may in great working order but if you the operator have no idea how it works it may yield unsafe results. Now if the customer is ok with someone who just admitted to knowing very little about their firearm to complete the task, then do as you promised and take the time to do it right. At the very least, if you did make a mistake then you don’t look like someone claiming to be a professional but having no clue.
