Home Forums Hunting & Shooting Ask The Gunwriters OAL vs Pressure?

Bludog,

TXram already provided my answer, and very well.

We're not talking about loads in chambers with different-length throats, we're talking about loads in the same chamber.

There was a time that I got to puzzling on this issue, and concluded that I was never going to get a full and satisfactory answer.

To fully explore seating depth and leade, I think you'd have to test various combinations that would require reaming the leade to different lengths.

It's just not an experiment that you can do on the cheap.

Meanwhile, for a fixed length of leade, I'm pretty sure Mule Deer is right.

The net capacity drops with seating depth about twice as fast as you might expect. Remember, that when bullet motion begins, little powder has burned, thus the case is fairly full of an incompressible solid, namely the propellant itself. For example, in a .30-06 with a 180 gn spitzer, the net case (below the bullet) is only about 40% gas. In this example, seating the bullet .04 deeper drops the gas volume about 2%. One would expect, then, the pressure to rise about 2%, but ideally it should rise a bit faster because when the burning propellant is confined to a bit smaller case, the pressure rises faster which in turn causes the burn to proceed a bit faster. QL calculates a pressure rise of 2.4% for the 2.1% drop in the initial gas volume of this example.

As mentioned earlier, QL's assumption about the initial bullet motion is very crude. MD has pointed out that in practice the extra jump in fact slightly lowers pressure a small amount. The explanation is "the bullet engraves more easily the faster it's going." I find that explanation wanting, but it's in the right direction.

The bullet will jump the leade at under 1000 psi. Let's say the jump was 1 mm (.040) but it was increased to 2 mm. At first, the acceleration is about 1000 psi over 1 mm. Reported bullet engraving force varies greatly, but 4000 psi will do for this. So, after the 1 mm jump, the bullet will decelerate at a net pressure of 3000 psi, meaning it will grind to a halt in the rifling after (very) roughly 1/3 mm. If the jump had been 2 mm, the bullet would halt after about 2/3 mm for a net increase of 1/3 mm, call it .015. Once the bullet is halted, QL's assumption becomes valid. So, instead of a 2% rise in pressure, we're looking at some faction of 1% drop in pressure, which is what the chronograph should be hinting at.

The back of the envelope is a fun place...

Question for those more knowledgeable than I -

Does the friction of the neck on the bullet really act to reduce the case capacity in practicality, or (without taking into account crimping, just normal neck tension), is the practical case capacity still mostly determined by max OAL (in other words, when the bullet touches the lands)?

I guess, to put in in a statement, my simple mind would think that seating deeper or longer would have minimal effect case-capacity-wise on pressure since the tension of the neck holding the bullet is not that much - much less than the force required to engrave the bullet into the lands. Therefore the bullet would easily move forward to the lands prior to much pressure being built in the case, so that seeting deeper would not have that much impact on pressure, not in the way that one would normally talk about case capacity vs. pressure.

Thanks guys!

I think you have mixed 2 or 3 separate issues here within your synopsis.
1. The case volume being reduced by a bullet seated more deeply
2. The amount of leade after an overall length is determined.
3. Neck tension effecting pressue

Begining with the last one, It would have a lesser impact though I agree that I too have noted a slight increase in pressure from older loads that have bullets more tightly seated in the necks. I belive this is due to increased resistance to release the bullet which permits the presure to rise above where it might be in a freshly loaded round. This is the very reason why bench shooters are seen carrying trays of primed cases around whereby they seat the bullet fresh before shooting to minimizs any "memory" and make as uniform as possible, the neck tension on the bullet.

The answer to #1, is not complicated. Although seating depths may reduce "volume" in a case when using heavy bullets especially, they powders selected and the charge weights are reduced to balance the volume of gas and peak pressure generated so as to approximate the same pressures generated when using shorter bullets in combination with faster powders.

Item 2, is an individual thing pertaining to a particular rifle. That is why reloading manuals are informational only as you chamber cannot be expected to replicate the test barrels used to compile the information. As JB stated, the pressue will increase faster with a long leade which is demonstrated by the little .458 cartridge in relation to its bore size. This cartridge has a small powder capacity and a very long leade for a factpry rifle so as the bullet travels the leade, the pressures rise to the same peak levels for any other high velocity cartridge chambered for the same action.

To demonstrate this further, the common practice of rechambering a .458 to the LOTT version which holds a case .300" longer with commensurate increase in powder volume, you will note this is achieved by shortening the leade section but in the end, the same or similar pressures are generated for both cartridges in a similar action.

John

The gist of my calculations was that the extra jump would allow the bullet to jamb further into the rifling before halting. The resulting small increment in volume behind the bullet would explain lower peak pressures.

Unfortunately for my argument, another quickie calculation -- one including time -- shows the problem is more complicated. With even 1000 psi propelling the bullet across the jump, it will take at least .1 ms to hit the rifling. A QuickLOAD pressure trace shows chamber pressure rising a few 1000 psi in such a time interval. If so, the chamber pressure may be high enough once the rifling is hit to prevent the bullet from slowing greatly.

So, this problem might not be answered by simple, back of the envelope calculations. A more complete math simulation of the problem, such as the Army is now doing, would likely be required to better understand the effect of seating depth on chamber pressure (and muzzle speed).

John, I'm sorry if my post was confusing, I did not mean 2-3 separate issues, but rather 1 rifle/throat length, 1 load, same neck tension, just varying bullet seating. As previously mentioned, two things affect pressure - 1) amount of leade and 2) amount of case capacity, both of which are due to how far out or how deep the bullet is seated. My question is in regards to 2) amount of case capacity - with a fixed leade, and normal neck tension, does seating the bullet deeper or further out really affect the "case capacity", or is the neck tension so little resistance to the bullet moving that it has no practical effect on the case capacity and thus volume (disregarding the effect of the distance to the lands changing the force required to engrave the bullet)?

As 2535 posted - "The net capacity drops with seating depth about twice as fast as you might expect. Remember, that when bullet motion begins, little powder has burned, thus the case is fairly full of an incompressible solid, namely the propellant itself. For example, in a .30-06 with a 180 gn spitzer, the net case (below the bullet) is only about 40% gas. In this example, seating the bullet .04 deeper drops the gas volume about 2%. One would expect, then, the pressure to rise about 2%, but ideally it should rise a bit faster because when the burning propellant is confined to a bit smaller case, the pressure rises faster which in turn causes the burn to proceed a bit faster. QL calculates a pressure rise of 2.4% for the 2.1% drop in the initial gas volume of this example."

I guess my question is that although this is the "theoretical" impact of reducing case capacity, is this the "real" impact? Or is it less than this due to the bullet starting to move forward prior to much pressure being developed?

It's not even theoretical! It only applies if the bullet doesn't move until some nominal engraving pressure is reached--which is what QL assumes. The initial bullet motion is much more complicated, and the result is the pressue drops a bit. Just why it does isn't obvious to me (in case you hadn't noticed).

Another variable I suspect exists, especially with ball powders, is:

greater powder compaction -> less surface area for primer flame contact -> slower ignigtion -> longer time to peak pressure -> more volume available when peak pressure is reached -> lower overall peak pressure.

In pressure tests I did some time ago, I noted occasions where an increase in powder charge (generally a ball powder) might maintain (or even decrease) peak pressure while increasing velocity. My assumption was that ignition was slowing, creating a longer, slightly lower pressure curve.