In MOST rifles the Berger VLD's shoot best jammed, but it's not universally true.
This article by Berger's Eric Stecker explains it. They used to recommend jamming all VLD's and just accepted that some rifles couldn't shoot them. But feedback from customers told them there were many of those that could be brought in by adding some jump.
Writing in the 1995 Precision Shooting Reloading Guide, the late Dan Hackett described a 40X in 220 Swift that he could not get to shoot 5-shot groups to better than about half an inch. He relied on the conventional wisdom to load 0.020" off the lands, another one of those things that work with a lot of rifle and bullet combinations, but not all. Then one day, when switching bullets, he turned the micrometer adjustment on his seating die the wrong way, going 0.015 in instead of out, as that bullet needed to be 0.020" off the lands. So he was seating 0.050" off the lands instead. He got 20 rounds loaded before he noticed the error. He considered pulling the bullets to reseat them but decided just to shoot them for trigger practice instead. To his astonishment, he got two 0.250" groups and two true bughole groups in the 1's.
Item number 3. in this old post describes and experience by Somchem with load development for hunting rifles, a service they offered a couple or three decades ago. Note that the rifle they describe is one with a worn out throat, yet by getting back off the lands far enough it became the most accurate hunting rifle they had ever developed a load for. That is followed by a copy of Randolf Constantine's old Precision Shooting article on Audette ladders.
There are complex dynamics at work in a firing rifle. Three observable factors seem to reveal themselves. First and best known is the muzzle swing flat spot the Auddette ladder identifies. You can read Constantine's description at the end of the page in the last link. In
this old thread on another forum starting with post #13, I helped a shooter analyze his Audette ladder, which looked pretty scattered but which resolved with the help of a few tools. Today, the
OnTarget's TDS software will do that for you.
Second is
Chris Long's Optimum Barrel Time (OBT) theory and the OBT round robin as Dan Newberry has been citing to explain his
Optimum Charge Weight (OCW) load development concept and theory for some time. This is the notion of a pressure wave traveling at the speed of sound in a barrel. You can actually see them superimposed on the pressure wave in a strain gauge pressure plot.
Third, the velocity node theory has made more noise lately on YouTube and in the various shooting forums. I first heard of this in the same 1995 Precision Shooting Guide in a section by Dave Milosovich (page 91), but the nodes were apparently already well-known to benchrest shooters when he was writing that; just not to me before then. A number of people shoot ladders based exclusively on these multiple flat spots in velocity that occur as you work a load up, and in the few instances where I have examined their plots closely they appear to have spacing that is reasonably similar to the uneven spacing of Long's calculated OBT nodes, so they may well be caused by interaction with pressure waves, but have the advantage that you can measure their location with a chronograph where the OBT nodes have to be calculated and that calculation can be off a couple of percent due to speed of sound differences in different kinds of steel.
I believe changing bullet seating depth affects the timing of the pressure wave and therefore the timing of the phases of the wave relative to bullet position in the barrel. However, like everything else in interior ballistics, it isn't that simple. Because a case neck expands open to release the bullet at firing, there is a brief period during the bullet jump that allows gas to bypass the bullet until it obturates the bore. You can see a brief stall in pressure rise on a sufficient resolution pressure plot. There are some in the late
Dr. Lloyd Brownell's 1965 DuPont funded study of pressure along with an explanation of gas bypass and why lack of it increases pressure when a bullet has no jump or very little (up to about 20% more pressure with a bullet in the lands rather than back from the throat by 0.030").
In the meanwhile, something Chris Long's theory and Randolf Constantines explanations both missed is that barrel "harmonics" do not determine the angular bend location of a muzzle at the moment the bullet leaves. There is harmonic ringing after it is gone, to be sure, but former Lawrence Livermore National Laboratories engineer Varmint Al has done analysis showing that
recoil moment and pressure distortion of the barrel actually cause and determine the timing of muzzle deflection during firing.
As a result of all the above, I don't any longer believe that any one of the various tuning theories is comprehensive. I believe peak accuracy occurs when the flat spots in an Audette Ladder due to muzzle deflection and a velocity or pressure wave node are coincident and that tuning seating depth and powder charge and type (different burn rates and priming affect barrel time) are all working toward that objective. So if you want to reach your rifle's accuracy potential, you have your work cut out for you arriving at that sort of synchronized interior ballistic performance.
Mind you, the success of all the above depend on you being able to prime properly (-0.003" reconsolidation of the primer in most instances), and load the round to be concentric about its long axis, and that you can control not only powder charge weight but its water content and its packing density in the case. It assumes you have a rifle that is not in need of lug lapping, recrowning, new bedding or any of the other usual accuracy basics. So all the ducks have to be in a row.
"First contemplation of the problems of Interior Ballistics gives the impression that they should yield rather easily to relatively simple methods of analysis. Further study shows the subject to be of almost unbelievable complexity."
Homer Powley
