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Yes, I stated my question little bit wrongly. What I was looking for, was what kind of difference an average shooter should expect with 4" and 6" because of different sighting radius on 27 yards.
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Let’s take the shooter and the subjectivity out of the problem and see if we can get an answer based on just the geometry involved.
Assume that a 6” barreled gun has a sighting radius of 8” and a 4” barreled gun has a sighting radius of 6”. I didn’t bother to go down to the safe and actually get some revolvers out to measure, but that can’t be far off. Assume further that you have the sights misaligned (doesn’t matter exactly how) by 1/25th of an inch (.04”) – i.e., the front sight is .04” off of what would be perfect alignment - when the shot goes off, and all other factors, including (most importantly) the shooter's ability to align the sights as the shot breaks, regardless of barrel length, and the inherent accuracy of both barrels, are equal.
In the case of the longer barrel, the sighting radius of 8” is equivalent to a circle with a diameter of about 50”. There are 360 degrees in a circle and 60 minutes in a degree, so that’s 21,600 minutes of angle (MOA). Each MOA, then, is equal to a distance of .0023” on the circumference. The sighting error of .04” would then correspond to 17.3 MOA, which results in a “miss” of about 4.3” at 25 yards, using the well-known rule of thumb that 1 MOA = 1” at 100 yards (and therefore ¼” at 25 yards).
For the shorter barrel, the circumference is only 38”, so using the same math the same sighting error of .04” corresponds to an error of 22.2 MOA, which would translate to a “miss” of 5.6” at 25 yards, or a miss that is about 30% larger than we had with the longer barrel.
The entire system is linear, so the 30% would apply to all amounts of sight misalignment and all ranges.