Why does one Lot of ammo perform better than another in a dynamic system? It is not just all about quality control and low ES and SD. A particular lot of ammo may more closely match the harmonics of the rifle system better than another Lot.
However, the harmonics of many rifles can be adjusted or tweaked to allow different Lots of ammo to perform in an excellent manner, not just one specific lot.
If this was not true then a Lot of ammo that performed well in one rifle should perform just as well in any number of rifles. Singling out only one variable of a dynamic system does not provide a complete picture of what is truly happening.
It's indisputable that high levels of quality control are required in the production of good ammo (the alternative would be low level quality control?). It's correct, however, that low ES and SD don't guarantee results. There's much more to excellent performing .22LR match ammo than MV characterisitics.
Why aren't ES and SD the only factors that determine performance?
The first and most obvious reason is that unless the ammo is chronographed, it's not possible to know anything about ES and SD. Since these are not guarantors of excellent precision/accuracy, it's not possible to confirm anything about MV characteristics by target results alone. To put this another way, a lot of ammo with a smaller ES and SD may be outperformed by a lot with numbers that are larger.
What are other important factors involved in performance?
The bullet itself is an important component in the performance of the cartridge. It's a considerable challenge for manufacturers to produce soft lead .22LR match bullets that are uniformly perfect. They can vary in size (
e.g. diameter and length) and weight. They can vary in uniformity of the all-important heel -- an imperfection in which can contribute to an unanticipated trajectory/flight path.
In addition to these, and sometimes associated with them, is the bullet's center of gravity. Unlike jacketed CF bullets, soft lead projectiles like .22LR bullets and lead air rifle pellets are much more difficult to manufacture with little or no Cg offset. Without actual testing, it's impossible for the average shooter to determine which lots have more or less Cg offset. Cg offset may help explain why some lots of the very same variety of ammo --
e.g. Center X or Midas + -- will perform differently as distance increases, even when their performance is virtually identical at closer ones. In other words, two lots of CX may shoot equally well at 50 but one of them may have more group dispersion as distance increases.
Some lots of the same variety of ammo -- CX, M+, Match, or Tenex for example -- are better than others because they have more consistent components such as bullets and casings. Some have more consistent priming, which is usually a greater source of MV irregularity than propellant levels because the "powder" volume is easier to get right in production than priming. Some lots also have more consistent loading, which is the process of putting together the primed and powdered brass with the bullets. Depth of seating and crimping consistency are important for accuracy performance. When crimping is inconsistent, all bets should be off.
The factors outlined above are all important to .22LR match ammo that performs well. Not every lot of even the same variety of ammo is equal. Unknown and unseen differences between lots help explain why there are differences in performance from lot-to-lot. They have nothing to do with differences in average MV.
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Regarding matching the "harmonics of the rifle system," it's often given as an explanation for things that are not obvious or are easily misunderstood. It's a broad overgeneralization to claim that one lot "may more closely match the harmonics of the rifle system better than another Lot".
Rifle harmonics are not not simply about MV or MV averages. Most .22LR match ammo lots have considerable overlap in ES (they are for the most part nominally in the 1050 to 1080 fps range) so there aren't great differences in harmonics to be found there.
While rifle harmonics generally refer to the wave motion induced in the barrel with each shot, vibrations that change minutely with ammo of different velocities, there are other important factors that can significantly affect a rifle's harmonics. Consistent harmonics are what's crucial for consistent performance.
Beyond barrel length and diameter, both of which remain consistent, rifle harmonics are changed by other factors. Among the basics is the shooting bench, which must be very solid in order to provide the basis for consistent harmonics. Rifle-to-rest set up and positioning must be consistent from shot-to-shot. When that's not consistent, neither are harmonics. In this vein, rifle hold is important and must be exactly the same from shot-to-shot. Most BR shooters don't touch the rifle except to activate the trigger. Removing the element of hold is the only way to come close to being consistent.
Differences in the factors referred to above may explain why some shooters who lot test at testing facilities where barreled actions are shot in a vise/fixture find that results at the range may not match those obtained during testing.
How are harmonics of a rifle tweaked? Shooters may try things like changing the action screw torque, making them looser or tighter or making the front screw tighter or looser than the rear. They may try to change their grip, trying a tight, "death" grip, or a looser grip. They may add or alter a weight on the barrel (not necessarily but often a tuner).
Some of these methods may work. The important question is do they work consistently? None of the will improve a poor shooting lot of ammo, jcust as a top tier rifle/barrel can't make inconsistent ammo into something else, like wine out of vinegar.
Barrel tuners are the most important tool for using harmonics to advantage. Because most lots of match ammo have a great deal of ES overlap, once a shooter has made correct tuner adjustments, little if any further adjustment is usually necessary to optimize the performance.
It's worth repeating that tuners give a relatively small level of improvement to ammo that already shoots very well. They don't change poorly shooting ammo into gold. They don't have the magic that will shrink groups by half or other similar claims that are often posted in fits of misinformation. They don't turn average, mass produced factory barrels into something else. They will not make an average rifle shooting average match ammo into something else.
While the most serious competitve BR shooters use custom barrels with specific chambers, it's worth keeping in mind that none of these rigs can outshoot the ammo they are given. They cannot make a poorly shooting lot into something different. What they can do is have the potential to get the best performance possible out of match ammo.
