Finally had a day that was conducive to barrel indexing. We had a 55 degree 4 hour window. Was very windy with switches so the best testing would be done at 25 yards to eliminate the effect of wind. Our normal rimfire range was closed do to berm enhancement. So we ended up on the pistol range with its rickity benches.
When barrel indexing, testing is done not to find absolute accuracy but to look at how the groups disperse. With a good barrel there is relatively little dispersion around the clock face. With a mediocre barrel groups can disperse as much as 6 inches at 50 yards. The barrels that benefit the most are the mediocre barrels. With a mediocre barrel there is on point in the clock rotation where groups will be much better.
My Suhl was barreled with a good barrel that had been previously indexed and marked. When it was smithed that point was made 12 o'clock.
The starting point is pictured below.
What a 1/4 turn rotation looks like in the rail fixture.
What a half turn rotation at the six o'clock looks like.
Here is what the whole rig looks like when testing. The pistol guys probably thought Martians had arrived at their range.
The proofs is in the the groups. This group reflects 20 shots shot around all four points of the clock rotation. The extreme right edge was generated by the 9 and 3 o'clock positions. Strange looking group.
Conclusion the original 12 o'clock position provided the least dispersion.
A tuner dealing with the little vertical in the groups would be an interesting addition to this barrel. Indexing deals with the natural curve inside the barrel and how it relates to the harmonic pattern of vibration. Barrels are not straight inside. Barrel indexing aligns the natural curve of the barrel to get the best groups.