A cute trick I found while looking at flash hole uniformity. Clamp your Teslong borescope in the vice looking up (no mirror) so you can quickly look inside brass.
And it's not always a pretty sight. I discovered that I hadn't rinsed and dried mine after testing wet tumbling well enough - After a short tumble and rinse (since the outside looked good), I vibe tumbled with my usual walnut shells to ensure dry-ness. Except not. And some of that media stuck in there, and the remaining liquid was still somewhat corrosive....
Since the entire point of this was to get the brass uniform, and do the experiment of making it uniformly shiny inside, as if new - this was a relatively important and shocking find. Some of this stuff was NASTY, and would have utterly invalidated any test of combustion uniformity.
Here's a pic of the lashup (ignore the mess in the background! Creative Chaos, or just chaos).
What this does is make it easy - it's in focus, and with a finger near the end of the brass, you can avoid scraping the camera with it as you put it on there, the top of the vise sets the distance to the subject.
Like I said, NASTY. You can imagine what bad results this would have if I just reloaded it. Since I'm not going near full power, it probably would have been safe enough, just terribly inaccurate.