When I keep everything the same and increase bullet seating depth away from the lands, I see slower MVs over my chronographs. Consistently. I can't speak to QL as I don't own it.
What I experience is consistent with the loading instructions that come with the HAT bullets. Load up a maximum load with the bullet a bit off the lands. Then increase seating depth systematically while looking for a sweet spot. You'll never experience an excessive pressure load in this manner, as pressures go down.
Therefore MV goes down by increasing the seating depth, with the powder charge kept constant.
That is not only wrong, it is dangerously wrong and could get someone seriously injured or killed. I don't care if it came with bullets or not. It is completely inconsistant with the physics of what is happening in a rifle case when the powder is ignited.
The only way pressures go down by seating the bullet deeper into the case is if the bullet was touching the lands to begin with. However the strength of the effect depends on several parameters that vary with case/powder/bullet combinations.
Reducing the case volume while keeping the powder volume and ignition constant increases pressure. This is most obvious on straight walled pistol and rifle cases because the bullet diameter is the same as the case body so there is significant volume reduction for small changes in seating depth, but the effect is the same on all ammunition. Peak pressure can happen before the bullet even leaves the case with fast powders, and happens often with handgun ammo. With the slow powders used with heavy for caliber bullets in rifles, like the 180 Berger/Retumbo combination in my 7mmMAG, peak pressure will happen with the bullet about 3" past the throat.
The examples of reduced case volume and increased pressure have happened, some times tragically. There are numerous cases on record of semi automatic pistols being blown up, literally coming apart due to over pressure, from bullets being pushed back in the case. One casue of this happening in semi-autos happens when the gun is routinely loaded with the mag full and one in the chamber if the one in the chamber is the same bullet, or one of the two bullets on the top of the magazine, are chambered time after time. What happens sometimes, but not always, is that when it's chambered the bullet gets nudged back into the case a few thou. No biggie. Do it over and over and pretty soon that bullet can be pushed far enough into the case to cause a serious, or even destructive over pressure. It can also happen on the first time if the bullet doesn't have enough tension to hold it in position when the round is chambered. A routine caution to those who routinely CCW a semi-auto is to rotate the round being put in the chamber.
It also happens with rifle cartridges. However, in rifles, the effect of bullet seating depth is not linear and changes direction with a discontinuity happening when the bullet is very close to or touching the lands.
Starting with the bullet a significant distance off the lands, say a jump of 0.100", at a safe pressure, keeping the powder charge the same and decreasing jump, will reduce pressure because the combustion chamber volume is reduced. Peak pressure is related to the amount of powder burned and the volume in which it is burned. Burning the same amount of powder in less volume will increase pressure, burning it in more volume will decrease pressure, all other things being equal.
The phrase "all other things being equal" is important. If one continues to decrease the jump to the point where the bullet is touching, or very close to touching, all other things are not equal. Touching the lands, or being lightly jammed (nobody can consistantly load bullets that are "just" touching the lands), will cause a significant increase in peak pressure of around 7,500 psi. A max load for a bullet with .020" jump is well over max with the same bullet jammed into the lands.
Increasing seating depth from .020" or more jump to jammed into the lands will see a big jump in MV, and probably significant pressure signs, when the bullet touches and gets jammed into the lands.
Another example of all things not being equal is if the case neck is too long. A case neck that is too long will be crimped by the begining of the throat and act to clamp the bullet in the case until a rather substantial pressure has built up. The effect is to reduce the combustion chamber volume by holding the bullet in place while the powder continues to burn. There have been some rifles blown up by this phenomena.
Finally, powder combustion rate is a function of heat and apressure. Powders have heat/pressure operating reigons in which their combusion is stable. Increase the volume, or temperature, or both to move the combustion process out of this stable performance reigon and the combustion process gets erratic and unpredictable. Some pistol powders exhibit this phenomena more than rifle powders. W296, for example, is quite unstable if the pressure isn't high enough. In .357 magnums there is sometimes the warning to not use reduced loads with W296 and only one load, the max load, is given. Unique, a favorite powder of those shooting .45 LC in revolvers, operates best at relatively low pressures and can get erratic if pushed too hard.
Finally, there is the issue of detonation that can happen when trying create reduced loads for subsonic bullet velocities, but that is unrelated to seating depth.
And this just begins to touch on the complexities of what's happening in the brass during powder combustion.
Fitch
