[Shoney]

We need to have some terms defined for common use.

The space ahead of a rifle's chamber just before the rifling starts is called the "leade," or throat, or "freebore" and is really the bore of the barrel with the rifling reamed away. The leade varies considerably in length and in the angle at which the rifling is cut, depending on the different factories, cartridge designers, and barrel and gun makers.

I have come to believe that most high production rifle makers us a leade that will be about 1/32nd of an inch (0.030”) from where the longest heaviest bullets per caliber are seated. I have seen some weapons with the freebore which allows almost half an inch of bullet travel before it contacts the rifling firmly.

General wisdom has told us that the “ideal distance”, from where the bullet is seated to where the bullet makes contact with the rifling, should be a gap of about 0.030”. This allows the pressure to build very smoothly and steadily even as the bullet hits the rifling. Pressure remains safe (normal) throughout the powder burn, and the velocity obtained is "wxyz" feet/second.

Seating the bullet deeper into the case, like most factory ammo, allows more travel before it hits the rifling, and permits the bullet to get a good running start. Powder gases have more room in which to rapidly expand without resistance, and the pressure thus never reaches the "normal" level nor does the velocity; as compared with the same powder charge at 0.030 off.

The saying that the “bullet must be seated at least one diameter in the case” is primarily for hunting rounds. If you have less than one diameter in the case, the bullet can easily be pushed off center line with the bore as it is being chambered. For target use, it is not a problem when feeding cartridges one at a time, provided they have been protected from distortion in the trip to the range.

When the bullet is not concentric (does not align with the bore). There is little chance of it going sideways down the barrel as some people may joke, but it will rattle back and forth in the leade, and can enter the rifling askew by a very small margin, enough to cause the bullet to wobble in flight. The same thing happens when you seat the bullet too far from the rifling. A longer barrel will not corret the wobble.

When the bullet is seated to touch the rifling, it does not move when the pressure is low; and not getting a run at the rifling as did the other bullets, it takes a greater increase in pressure to overcome the friction of the rifling and begin to move down the barrel. The expanding gases have less room than they should have at this time in their burn, and the pressure rise is both rapid and excessive. Velocity is higher than the Short and normal length cartridges, but could be at dangerous pressure if the powder load is at or near max.

Many rifles deliver their best groups when bullets are seated just touching the rifling. Seating bullets into the lands can be done quite safely if the reloader will reduce his charge by a few grains. The lighter load will still produce the "normal" velocity and pressure.

Barrels with several thousand rounds through them have been noted to fall off in the accuracy, using their standard loads. Hot gases, unburned powder, and incompletely burned powder particles, from the all the shots fired through the barrel, erode the throat and thus increase the distance a bullet must travel before contacting the rifling. By loading longer bullets and seating them farther out so they'll be closer to or touch the rifling, and adjusting the powder charge, can improve accuracy again.

In addition to the seating depth, factors such as primer, bolt alignment, barrel length, tightness of bore, height of the lands, bullet, brass, powder, weather, elevation above sea level, and more all contribute to accuracy.