Most efficient process to diagnose and fix problems with loss of zero or change in precision?

[Macintosh]

Thanks Marbles, thats similar to what Ive arrived at.

 

[Macintosh]

Folks keep trying to take it back to the shooter. I think its plenty to simply say that if the shooter previously shot the same gun/load and got it to group, or had it zeroed, that shooter-error is not the low hanging fruit on a failed zero-check. If people are talking about misses at longer range, I’ll DO a zero check at 100 yards to eliminate a shift versus shooter error as being the culprit—based on my previous large-round-count zero, either the zero-check shots will land within the expected group size on the point of aim, or they wont. If theres any further question about a gun not grouping (with ammo it previously grouped well), handing the gun to a known good shooter of that cartridge or similar will suffice to determine if its shooter error or equipment problem. Im really asking about diagnosing equipment problems only—Im assuming a proven shooter.

 

[Rippey715]

New miles to matches podcast 23:00 ish

 

[Macintosh]

I listened to the miles to matches podcast referenced above. It's really focused mostly on ultra-high-volume shooters and reloading issues, and lot to lot variations in ammo components, plus when to consider a barrel shot out. Only at the very end do they begin to address a process for identifying a scope issue or some other problem like that. It's on topic, although I would not say it squarely addresses this issue in the way I see it come up most often for most people. https://podcasters.spotify.com/pod/.../EP107---When-Good-Goes-Bad-e2hkj9b/a-ab4205l

However, this podcast just dropped, which squarely addresses this topic.

The step by step process detailed in the S2H/FF podcast (as I took away), is heavy on the "setup before a problem" to anticipate and avoid issues in the first place, then has an extremely simple set of steps to address the most likely culprits of fasteners loosening or connections not being tight enough (most likely), then scope failure (also moderately likely), then a gun problem (much less likely). The troubleshooting process itself is actually very similar to the process outlined in the miles to matches podcast, although they did not get into nearly the detail.

I personally see this coming up 99% of the time with people who have NOT already put a lot of assembly prep into avoiding this situation, so my own step by step would be to:
1) Assuming no existing witness marks to check for movement--if zero shifted or precision with same ammo decreased measurably, my first step would probably be to tear the whole gun and scope/mounts apart, examine crown, bedding, barrel channel and action for obvious issues, degrease, reassemble with threadlocker and torque wrench to max torque, and apply witness marks to all screws and scope/rings and action/stock.
2) rezero off at least a 10-round group to the nearest click, and record extreme spread of 10-round group. If it adjusts and zeros normally and precision with same ammo is back to normal, it's probably fixed. And, you have a pretty good extreme spread for dispersion and a good zero to measure any future shift...any future group using the same ammo lot that is significantly above that extreme spread, or lands outside an aiming point the size of that ES, is a loss of zero or a change in precision, so make sure to keep a solid baseline like this after any future ammo or gun changes.
2b) Some folks do a drop test here, some just shoot it over time
3) If it wont zero/stay zeroed or dispersion is still above where it was, and witness marks indicate no movement, swap to a known good scope and repeat. If it goes back to normal, it was almost certainly the original scope.
4) If it doesnt, it was the gun. If there is no muzzle burr and barrel is torqued, and round-count is low-enough that the barrel cant be shot out, bedding is very good, and a cleaning doesnt fix it, then it's a rarity and may need an exorcism by a qualified gunsmith to find a hidden, low-likelihood problem such as the blind fasteners on the gun mentioned in the podcast. I've never personally seen such a change in a gun that wasnt fixed by steps 1-3 though.

That's basically the gist of what I have been doing over the past several years. I had a few issues with some guns losing zero and using a system like this was able to quickly identify it as a rail/ring loosening issue and others as a scope problem, and when addressed that way it has completely eliminated the problem with a minimum of time, fuss and ammo because its really pretty simple. So far, of the guns I've worked on in this manner, it has fixed 100% of the problems I've ever seen with loss of zero or a change in precision.
I'm sure there are situations I haven't run into yet, I have been assuming there are other issues and ways to address that people have found, so curious what others do in this type of situation and how they see the above system working for the problems they've had? If you have a problem with it, is that becasue you think it doesnt actually isolate the problem, or is that because you are thinking of other problems that arent included here? If so, what other problems have you run into and how did you diagnose and solve?