6 0'clock hold is a paper target bullseye shooters sight picture.
It works perfectly everytime for the known distances of 25 & 50yrds of what Bullseye pistol shooting is.
The white paper under the black 'center' of the standardized targets makes for a nice sight picture.
There's no guessing about where the center of the black is no matter the diameter, you just place the round black target center on top of the gun site picture.
If sighted in correctly, they go into the center of the black.
In reality the gun is shooting high at that distance for that diameter 'center' on the bullseye target.
If you are shooting the 50yrd slowfire, and use 6 o'clock hold you are 'aiming at the bottom edge of the black of the target.
You are aiming approx 5 inches low of the X to begin with (I forget the exact diameter of the black on those now).
But your sight adjustment is so that the rounds hit high,,in the center X of that target.
You can see how a pistol/revolver thus sighted could be all over the place at other distances if shot using a center hold or using different 6'oclock holds.
If your instinct is to shoot 6'oclock sight picture on targets,,that's most likely what you'll do in the field also.
Center hold (they used to call it a 'Navy Hold as opposed to the 6 O'clock Hold) does away with the variable of the size of the 'bullseye' or lack of any.
You shoot at the exact point you want to hit. Sounds simple & logical enough!
You pin point the spot you want the shot to hit. You don't blot it out with the front site, but instead that tiny spot sits on the top of the site picture.
Like the old 'draw a find bead' terminology in shooting,, you shoot right to the mark.
You can blot the center out with the center hold in bullseye shooting using a bead front sight and some shooters using that style hold and sight equiptment do so.
That's trying once again to take advantage of the targets themselves and the contrast to up your score and nothing wrong with that. But in the field, blotting out a target point is usually a missed shot unless it's a very close one.
Center hold is real world practical,,6'oclock is mostly a paper target bullseye sighting advantage using known ranges & target diameters.
Most of the people I see at the range now crank a piece of cardboard out to 15 ft and blast away 2 handed.
Seeing that they hit the piece a few times that is near the size of Buick car door, they're satisfied w/ high fives all around.
I can't imagine that any sort of sight picture is part of their game, but even up close it should be if they bring the pistol to eye level..