The method you described still works with the M3 and certainly works well for pellets, although my preference is to set the hammer spring slightly under the peak velocity plateau for a given regulator pressure and then fine tune down just a bit with the valve adjustment if tuning following your method.
The other line of thinking for tuning (especially with slugs, but for pellets too) is to run the valve wide open, use a higher reg pressure than needed for your desired velocity, and dial in the velocity by reducing the hammer spring which will end up well back of the peak velocity for that regulator pressure. The idea behind this tuning method is that using a higher than needed reg pressure results in a very short valve opening time for a short pulse of higher pressure air for better efficiency. The higher than needed reg pressure also helps to close the valve quickly.
One question when tuning this way (higher reg pressure with reduced hammer spring) is shot count compared to a tune as you described, where the regulator is set just above your desired velocity. Some say the higher reg pressure with reduced hammer spring is more efficient because of shorter valve opening times which results in better air efficiency, where others say the method you described will get more shots per fill since the regulator pressure is lower with that method and from a full fill you have a wider working range before the rifle falls off the regulator.
I recently tuned my new M3 for slugs, and while 120 bar on the regulator was enough to plateau 15fps above my desired velocity, the rifle was much more accurate with the regulator increased to 135 bar and the velocity set 45fps behind the peak velocity using only the hammer spring tension and leaving the valve wide open.
Next test I need to do is compare shot count and accuracy at 135 bar on the regulator using 2 methods to reach the target velocity: 1 leaving the valve wide open and the velocity dialed back using the hammer spring as I have it now, and 2 setting the hammer spring for 5-8 fps under the peak velocity with the valve fully opened and then reducing the velocity to the desired velocity only by closing the valve. Method #1 seems to be the most recommended for slug tunes, but I want to see what effect method #2 has on accuracy, velocity spread, and shot count.
EDIT:
Just got back in from trying the tune both ways with my slug tuned M3 (max valve / reduced hammer spring tune vs max hammer spring / reduced valve tune.)
With the valve fully opened to about 4.5 lines and the regulator at 135 bar, the peak velocity was 1005 fps.
First I tried my current method of tuning which was leaving the valve at about 4.5 lines (full open) and dialing the HST back to the max accuracy velocity of 960 fps. Then I did a shot count test from 200-150 bar. Shot count was 35, extreme spread was 9fps, SD was 2.5fps.
Next, with the valve still at 4.5 turns full open I adjusted the hammer spring for 1000fps (just under the peak velocity) and reduced the velocity to 960 fps using only the valve adjuster. Shot count from 200-150 bar was 34 shots, extreme spread was 13 fps, SD was 3.4 fps.
While the numbers (shot count, ES, SD) were close, the SD/ES was marginally better with the max valve / reduced hammer spring setup. The accuracy was also better with the max valve / reduced hammer spring tune. Also, the sound of the rifle seemed to be identical with both setups, which makes sense as air use / shot count was pretty much identical between the two setups.
I suppose it all boils down to which tuning method produces better results at the target. The Impact gives you multiple ways to reach the same goal, which is either a good thing or a bad thing depending on how you look at it. As evidenced by my test tonight, 2 different tuning schemes can produce nearly identical shot count / velocity / ES / SD numbers but have a noticeable difference in group size... which means it takes more air, ammo, and time to test all combinations to find out what's the most accurate.
