Dry firing does not help to diagnose shooting errors as much as
- mixing dummy rounds in
- having a 3rd party observe you while you shoot
At 30ft, there are not a whole lot of things you can screw up, so with observation and using dummy rounds in the mix, it's relatively easy to spot, and most times correct.
In the final analysis, if the firearm's POA = POI, you are disturbing the sight picture immediately before, at or just immediately after the trigger breaks. We generally do these things because of
- recoil anticipation
- lack of proper follow thru
- attempting to "look over" the gun's sights to see where the shot hit
- attempting to pull the gun down in preparation for the next shot (very similar to recoil anticipation, but is really a result of wanting to maintain sight alignment with faster shooting).
The corrective actions for the above would be (again, the assumption here is that the gun is 100% functional, and accurate)
- stop flinching.
This can be minimized by doubling up on hearing protection (not as loud muzzle report), shooting outdoors in daylight (less muzzle flash, not as loud)
- stop anticipating recoil. Mixing dummy rounds in with live rounds will help you to see if you do this.
- practice good use of the shooting fundamentals, and in particular, follow thru.
- don't spend too much time trying to see where you hit.
- don't be in a rush to make the next shot. Each & every single shot should go thru that mental "pre-flight" checklist of the fundamentals all the way up to follow thru. Make every shot count, take your time.