seating depth and pressure effects

[rockwind1]

i guess it is pretty well accepted that if you jam your bullet right up against the lands, you will have more pressure than otherwise, but is that fairly consistent as you move your bullet further and further away from the lands? like less and less pressure as you get further and further away from the lands? or is it a more on/off effect?

and what causes the pressure if it is up against the lands vs off?

 

[BoydAllen]

At the beginning of the rise of pressure, it takes a certain amount to get the bullet moving forward in the case neck, to overcome its inertia and the friction between it and the case neck. Then there is the force required to engrave the length of its shank with the rifling. When you jump these happen in sequence and the slight momentum that the bullet develops before it reaches the rifling probably reduces the amount of powder supplied pressure that is required to engrave the bullet. When you start out touching, or into the rifling, the inertia, friction in the case neck and engraving force requirements have to be overcome all at the same time, which causes a slight delay as the pressure builds up to that higher level and it also means that the bullet is taking more time to "get out of the way of the increasing pressure, which results in it going to a higher peak pressure, because the combustion takes place at its own pace.

I have done some fire forming of PPC brass starting with the bullet at jam and measuring the case "headspace" for that firing and subsequent firings at the same powder charge but progressively less into the rifling. As the amount that I was into the rifling decreased, so did the case "headspace" which to me indicated less peak pressure. This is consistent with what we find experimenting with different distances into the rifling as a means of tuning. Once you get out of the rifling by some small amount case volume is not much different than touching or in the rifling, but as you continue to back out of the rifling you can reach a point in some situations where the decrease in initial case volume can increase the pressure, just like putting the same load in a smaller case would. The reduction of case volume causes more increase than the jump can compensate for.
Added a little later: I use the term jam to mean the longest that a bullet can be seated and not be pushed back when the round that it is a part of is chambered. It is a specific dimension that varies with the internal dimensions of barrels, the diameter and shape of bullets, neck thickness and hardness, difference in sized and bullet seated neck diameters and friction between the neck and the bullet. We usually find this dimension by seating long, measuring, chambering and then unloading the round, and then remeasuring. If the round got shorter than the second measurement is taken as jam for that particular set of circumstances. Other seating depths that are shorter can be referred to as so many thousandths off jam or shorter than jam. In the last few years shooters who have not been around benchrest competition have somewhat corrupted this definition and the internet has increased the effect. This is why I explain what I mean when I use the word.
When one is jumping I think that it is better to use some procedure to establish a nominal "touch" length and then describe seating depths by how much one is jumping the bullet, rather than continuing to use jam.

 

[rockwind1]

so does the bullet actually stop after moving from the neck when hitting the lands and then the pressure rebuilds until it moves again?

 

[Bart B.]

It takes at least 815 psi (60 pounds of force) to start a 7.62 NATO M80 147 grain bullet moving out of its case neck. One tenth to one fourth as much for most of our reloads.

Here are data military did on pushing bullets into rifling. The data on 7.62 M80 and Barnes bullet is interesting.

https://www.google.com/url?sa=t&sou...FjABegQIBxAB&usg=AOvVaw3TcYwBG1mp5z6sBpEf2ZFb

 

[jepp2]

I've no idea what the force required to push it fully into the rifling. Never found any data searching for that Maybe a couple thousand psi.

The closest I have ever seen is what is listed in the bubble help in QL.

 

[Bart B.]

The closest I have ever seen is what is listed in the bubble help in QL.

View attachment 1044683

Multiply QL psi numbers to push bullets into rifling by .0512 to get the pounds of force needed.

Updated my post with real info.

What the military test folks call the forcing cone (FC) is the throat or leade in SAAMI speak.

 

[Bart B.]

so does the bullet actually stop after moving from the neck when hitting the lands and then the pressure rebuilds until it moves again?

Yes, so thinks me.

Some reloads used in matches have little case neck grip on bullets. Bullets can be pulled out by hand easily, Only several ounces to a pound or two of force is needed. They'll stop for a few microseconds before the pressure pushes them into the rifling and the lands start engraving them. Then more pressure speeds them up and about .0001 second after the primer fires, they're fully engraved by the lands and continue speeding up until they're out the barrel plus an inch or two past the muzzle.

Other rifle bullets stay in place longer as they're gripped harder. Primers alone sometimes won't push them out of cases without powder.

Interesting stuff: https://www.shootingsoftware.com/pressure.htm