I have been getting a number of queries regarding how I identify a rifle barrel's sweet spot.

First you need a weigh scale capable of precisely measuring powder loads to 0.1 grain variations. I use a RCBS 10-10 scale and make sure I calibrate it before testing and I bump the pan downwards on each measurement to make sure the scale has not hung up.

I  measure test powder loads 0.1 grains apart which will be test fired to determine their group size.

I start with a general powder load being used for a specific bullet, caliber, barrel twist and powder being used. (i.e. 12.0 grains H108, or AA#9, or WC820 with 210 to 235 grain bullets in 1:14 to 1:11.5 twist .32 caliber barrels)
I usually start with a powder load to test that is a 1/2 grain less. I  load 10 powder loads 0.1 grains apart so the largest powder load will be 1/2 grain larger than my starting load.

I have found about every 0.3 grain variations there is a significantly smaller groups size where barrel vibrations are less. After testing the first 10 loads you should have three spots that are smaller (with two larger groups in between them). Comparisons of these smaller groups indicates if you have to go up or down with your powder loads to find the smallest group size. 

You can continue to test the entire potential powder range to find the smallest group. I stop when I acheieve 1/2 MOA from powder testing.

I then further refine this sweet spot by fine tuning the chamber's air space by opening the Weber seater by extending the plunger by 1/2 a twist.  I test the group size differences that occur by changing the air volume and put the plunger back to the point where I obtained the smallest group size.

Before I start doing the powder testing I first ensure my adjustable seater is set so the base band of my bullet is engraved by rifling.

So the smallest group size and the sweet spot is identified by testing powder loads which are then fine tuned with adjustments to the seating depth.

Dave, do you use a sled on any of your bench guns? If so, how do you determine the location of the sled in your tuning process.

Thanks Dave, appreciate your reply.

John I do not worry about the speed of my bullet for Schuetzen competitions. Rather I let the rifle demonstrate the speed which demonstrates the best sweet spot which provides the smallest group size. There is a sweet spot roughly every 0.3 grains of powder. Of course each sweet spot will not demonstrate the same level of accuracy so you need to then evaluate the differences in accuracy of the different sweet spots you identified. The very best sweet spot may be at a lower velocity in one rifle versus a higher velocity in another rifle. Firing groups is the only way to assess the point of least vibration achieved by a specific barrel when assessing different powder loads.

Each barrel demonstrates different harmonics. For the four different similar .32 caliber RKS barrels for which  I identified the best  sweet spot; the velocities varied from around 1,400 fps to 1,525 fps. 

The speed for one rifle's sweet spot will not provide the same vibrations when tested in another rifle, even if it has the same type of barrel.

It is also interesting that whenever I have identified the sweet spot for a rifle  and the smallest group sizes; the chrony does not indicate that I have achieved the smallest standard of deviation of  the velocity for the powder loads tested. I have found that the assessment of bullet velocities has not assisted me in identifying the best sweet spot. 

Once your standard deviations of velocity are less than 1% you cannot assess differences in accuracy from evaluating changes in SD of the velocity. I have seen this specified in the literature, but unfortunately I do not have a reference for you.

Most rifles have sweet spots that show better accuracy at faster velocities until you start exceeding the 1,525 fps since then recoil becomes more of a factor on group sizes.

Remember for this testing I have used spitzer bullets which are less susceptible to drift at these velocities and shooting at 100 to 200 yards. You need 1,525 fps for 300 yd accuracy.

Dave 
I'll describe my method it has some similarities and some differences. First I start with a load that I know is slow and work up, shooting over a chronograph, adding two clicks on my Harrel measure. I make note of my measure setting when the vertical stringing closes up and continue on. There is usually about three speeds where the vertical stringing is at it lowest before they start to open up again. 
I then go back to the settings that gave the lowest vertical stringing and start to test and fine tune each. Keeping notes over several days and weather conditions as to group size. Then the loads are evaluated I choose the one that has given the best average. Then I adjust my seating depth. My starting seating depth is always .065" When I adjust I start .010" low and work up .005" at a time. Keeping careful track of group size. Only when I have what I consider a successful load do I make note of the actual charge weight.

40Rod:

I see how you are finding your sweet spot by evaluating small powder variations combined with volume fine tuning. Yes I have noticed differences of group shape where a sweet spot provides tight clustered groups whereas non-sweet spots demonstrate larger shotgun patterns and/or string shaped groups.

I see you use a chronograph; but I am not sure what it is telling you.

Dave I use the chronograph to tell me the the bullets speed when I hit a sweet spot. I don't bother with the powder volume until I have determined my final shooting velocity. For two reasons changes in temperature demand subtle changes in the powder charge. The second reason is that with the Harrell click type adjustments when I dial to 92 1/2 I get the same amount of powder every time. The only time I use a volume measurement is when I fill out my load information on a match form or when giving a load to another shooter as even if the shooter has a Harrell measure like mine the drop at 92 1/2 is probably different.
  John I have never been able to get the first shot out of a clean barrel to go in the group consistently. I usually put two down the barrel before starting on a target. The same when I make a powder change I put one or two off target before starting on a group. I have found that when I change settings especially subtle changes the velocities take a shot or two to change reliably. I also agree that one must stay either supersonic or subsonic from muzzle to target my usual choice is between 1450 and 1480. 

40 Rod

Dave 
The Harrel Schuetzen / Pistol measure is the the best measure I have found for ball powders. The advantage to the click measures is that when you dial to a reading you will get the same drop every time. But it is not a substitute for good technique. You need to spend time with the measure to find the routine that will give you the most consistent drop. Mine is to bring the handle up to the stop firmly hesitate and then bring it down firmly so it produces a slight knock at the bottom. Then leave the handle down during the balance of the loading process. I know that this is the advice that almost everyone gives it is more about "feel" and habit getting a correct drop. You will have to develop it yourself but now I can keep a 10 shot string consistently within 10 FPS at this point the wind is much more your enemy than variations in velocity. 
One more tip I rub the inside of my plastic powder bottles with extremely fine graphite to the point that you can barely see into them. This helps to keep the measure or powder bottle from picking up static. I have observed on really dry days that the grains of fine ball powders will stick to the sides of the bottle. The graphite stops that.

40 Rod