so far I have 2 rifles that it has helped me with: concrete proof, nope--even if I had 2 sets of targets would you believe me? it would still be my word on what those targets were--before/after/with/wiithout/ 100y, 1000 yd/etc--- this is the internet after all.
in 1 case I have a "new" old stock rem mountain 700 in 30-06, it wasnt shooting really well (about 1.5moa) and I thought it was because of the light weight and the pencil profile barrel-- after the bore scope I saw it had LOTS of copper fouling in the bore (it has low round count on it but appears to be copper fouling faster than my other barrels), so I cleaned the snot out of it till there was no visual copper left, then re-shot my 1.5 moa load and came out with 3/4-7/8 moa --so I guess you could say it cut my group size almost in 1/2
2nd one was just for information and did not increase my accuracy--its my old arisaka rifle (on another thread) -- it shoots like crud so I scoped it to see if there was a lot of throat wear fire-cracking, copper build up, pitting or what ever that could be affecting the accuracy --- what I found was the bore is almost smooth inside, especially near the muzzle--I had no prior reference point to know that the accuracy has decreased so it was nice to know that it would be a waste of time to try to reload for it to see if I could get any accuracy out of it (this easily saved me more than the $50 it cost in dies/brass/bullets/powder)
on my other rifles, I can watch to see how much copper builds up in each bore, and I can watch for any carbon ring build up (if it matters) -- I did see 2 of my rifles with a carbon ring starting to form-- the others have no trace of a ring --good or bad, not sure but at least I can keep track of it to see if copper build up or carbon build up is affecting my accuracy
I feel it gives you a baseline to observe changes to see if they affect your accuracy, or pressure
The other reason I bought it is so I can scope a used barrel to see if there appears to be excessive wear--- I don't feel you can tell if a barrel will be accurate by just looking at it though as some rifles that shoot well, just look like crud
Is it 100% necessary? NOPE- is it a useful tool? YES in my eyes
You could loosely compare it to a leak-down and compression gauge used for an automotive engine-- do you need one to tell if your horsepower and performance has decreased? nope, does it help to diagnose exactly what is going on inside the engine that is causing your problems? yes in my eyes