I remember when the technique was called a Zipper. Then, it was later being discussed in training for instructors as 'vertical stringing', because
nobody in their right mind wants to find themselves in front of a jury explaining why they were teaching a technique called a 'zipper'.
Intentionally stringing shots vertically is probably best left to something introduced to at least intermediate level & experienced shooters, if not advanced.
Learning to ride the recoil impulses, controlling recoil and not letting it control you, while maintaining a solid grip, sight alignment and trigger press is one thing. Another thing is how a dynamic threat target may be moving, and how the neck/head is a really narrow and small target. I've seen shooters trying for intentional head shots on stationary paper threat targets put hits off to one side or the other of the head, above the shoulder.
A
moving paper target, even if the movement is occurring within a defined & expected manner (or path)
really increases difficulty ... and that doesn't even take into consideration potential
shooter movement in any given situation or set set of circumstances.
When I've introduced vertical stringing to an intermediate, interested & experienced shooter, I've usually done so as a
teaching tool, to better assess a shooter's skillset and mastery of technique basics. For example, does the recoil control the shooter in a new situation, or can the shooter automatically adjust and control the recoil while making accurate hits? Then, I slowly introduce shooter movement (forward/rearward, diagonal and lateral - starting from both left & right directions) while performing the drill, and changing the number of rounds being fired.
This is when most shooters start to realize that accurate fire means
aimed fire, especially out beyond 3 yds. When even moderately experienced shooters start missing COM hits between 5-9 yds, while they're moving but the threat target remains stationary, it usually gets their attention (and not in a good way).
I remember listening to a cop who had survived an on-duty shooting during which he'd been seriously injured. The shooting was later determined to have involved distances between the lone cop and armed suspect of 5-100 feet. The cop said that after he'd been seriously injured, and hadn't hit the attacker after having fired 22-23 rounds ... and was loading his last hi-cap mag into his .40 duty weapon ... he realized he had to stop 'shooting instinctively' and start
aiming. He did just that, hitting and stopping his attacker.
I also remember showing this technique to an experienced instructor and cop who had been involved in 2 shootings of his own (successfully stopping both suspects without being injured himself). The first thing he said after practicing the technique was that he wished he'd have learned it sooner ... but he's a
very experienced & skilled shooter.
FWIW, I remember some years ago when one of the larger state agencies reportedly did a study of their on-duty shooting incidents covering a number of years. They supposedly discovered that in just shy of 65% of the shootings reviewed,
both the officer and the armed suspect were in constant motion.
Bottom line? The totality of circumstances involved in any particular shooting incident can't be predicted. There's a reason these incidents have been referred to as unexpected, dynamic, rapidly evolving and chaotic occurrences.
"Head shots", and the neck/cervical spine, are going to be
extremely difficult to make on even a close stationary threat target. Being able to get an adequate flash sight acquisition and not jerk the trigger under fast shot strings seems pretty hard for many shooters of casual training and experience. People miss more than they expect even under controlled range conditions.
Learn, train and practice hard to master the basics, and then periodically test them under increasingly more demanding drill conditions (supervised by an instructor). Try some IDPA or similar local competition venues.
Don't try to run before you can consistently & successfully walk over difficult and uneven terrain.
Remember that each and every shot fired is going to hit
something, and bear in mind where the responsibility for the consequences of that bullet impact may be found to rest.
Just some thoughts.