I bought a digital borescope from Amazon for $46 that connects to a laptop or smartphone to view the image. It's like giving a colonscopy to your guns. I'm fascinated with what I'm viewing. Rifle barrels that I thought were clean and shiny turn out to have considerable copper fouling and erosion. They visibly need more work with brushes, patches and solvent. Now I understand why I can't get good accuracy from my Finnish M39 Mosin-Nagant. The inside of that barrel is so rough it looks like the craters of the moon. Every shot deposits more crud inside and there's no uniformity.