I have read that reducing OAL in a bottle necked rifle round can increase pressure. I have also read that it can reduce pressure. I read a post here at the CF awhile back by John Barsness about OAL and pressure, but I can't find it. I also found this article by Barnes on OAL.
http://www.barnesbullets.com/resources/newsletters/september-2007-barnes-bullet-n/

So, I thought I would come here and ask the Gunwriters, does reducing OAL in a bottle-necked rifle round increase pressure, decrease pressure or have little or no affect?

It decreases peak pressure, for two reasons. The longer "jump" of the bullet to the rifling results in a lower peak pressure, since the bullet engraves more easily the faster it's going when it hits the rifling.

Also involved is the "progressive" burning of almost all modern rifle powders. This means the pressure increases relatively slowly from the time of ignition. Thus peak pressure occurs when the bullet beyond the barrel throat, with very slow-burning powders as much as 3-4 inches.

Handgun powders are much faster-burning, and even regressive, meaning pressure peaks when they're first ignited. Thus seating them deeper allows more time for initial pressure to build.

This effect is slightly exaggerated in revolvers. The initial, quick pressure rise is relieved somewhat when the bullet passes the cylinder gap, allowing some gas to escape. When bullets are seated deeper it takes them longer to pass the cylinder gap, giving pressure more time to rise.

This is an excellent question, one which I have asked myself in the past, and John's reply makes absolute sense, except in one regard (that I may well be wrong about):

Increasing overall length, (i.e. seating the bullet less deep) I would believe, would reduce pressure with consideration to such procedure increasing the case capacity (given the same powder charge)until of course one got very close to the lands, resulting in the scenario John describes.

I think the enlightenment I gained here is in regard to John's description of progressive powder behavior, and when peak pressure is reached.

There's a lot of handloaders out there who believe they can gain a reduction in pressure by seating their bullets so as not to take up so much room in the neck/case.

Perhaps this needs some re-thinking. Or, perhaps there is a balancing act going on, and there is a point where this "gain" of seating bullets long(actually a perceived reduction of pressure)is negated by the bullet being closer to the lands.

John's contention certainly fits with Weatherby's technique of "free-bore" toward pressure reduction.

Tahnka,

Seating rifle bullets further out to "increase powder capacity" doesn't work the way you suggest, for the reason I previously stated: The faster a rifle bullet is traveling before encountering the rifling, the lower the peak pressure, because the bullet engraves more easily when traveling faster.

This has been demonstrated over and over again in pressure laboratories. You can demonstrate it yourself by shooting 5 rounds of a given load in one of your rifles, with the bullets seated out as close as possible to the lands--or even touching the lands. Next shoot 5 rounds with the same load with the bullet seated one turn of the seating-die spindle deeper, and repeat with another 5 rounds with the spindle another turn deeper. The average velocity will be slightly less with each batch of deeper-seated loads. (That is, it will be if your chronograph is accurate enough and light conditions are consistent.)

Muzzle velocity is one of the direct by-products of pressure, and the one most easily (and accurately) measured by home handloaders.

Myth buster.

I don't know the answer, I always went with decreasing OAL decreasing psi. Then I got Quickload and it indicates much higher peak pressure with shorter OALs (all else being equal). That does not make sense to me though. Having a greater volume when the bullet hits the rifleing would seem to decrease peak pressure to me.

Just curious (never used Quickload), does Quickload "know" the distance to the rifling? Or is assuming some constant distance? If the latter is the case, it would make sense that it predicts higher pressure with decreasing OAL.

Haven't used it in awhile, but I don't think it knows. I'm certainly not a QL expert though. What you are suggesting makes sense though, because you would have a smaller volume.

Don't think velocity is necessarily a good indicator of peak pressure. Total pressure while the bullet is in the barrel results in velocity. You can have a very high, but short, peak pressure and have a lower velocity than another powder that has a lower peak pressure, but broader pressure curve. That's how I understand it anyway.

Well I guess I should hold this one but ........


How does crimping affect all this????????

prm,

I suspect we are talking about two different things:

Velocity does NOT indicate anything about peak pressure in DIFFERENT LOADS.

It is, however, a very good indicator of peak pressure when the same load is fired in the same rifle. If a different seating depth shows higher or lower velocity, that does indeed mean that pressure is higher or lower.

Major edit... Yes I agree with that.

Ackley did some testing on long freebore systems, and found that it doesn't yield higher MV.

I think you really have two effects working in opposition to each other. Smaller initial case volume increases pressure, while freebore reduces it.

The bullet get most of its velocity while pressure is near its peak. So, while velocity is determined by the total area under the curve, for any given bullet and powder combination, MV is highly correlated with peak pressure.

You certainly have two opposing forces here:

- smaller expansion chamber = higher pressure (shorter OAL)

- more bullet momentum at time of engraving = less inertia to overcome = less frictional force = less pressure (shorter OAL)

It is my suspicion (based on the experimentation that John mentioned) that the first factor is the minor one in the overall pressure equation, with the latter being the major player here. So while reducing the combustion chamber does, indeed, increase pressure, the corresponding increase in bullet momentum when it engraves in the rifling reduces the pressure, which far outweighs the slight pressure increase caused by the smaller case capacity.

QL does not consider bullet jump or crimp. It assumes the bullet doesn't move at all until a specified bullet engraving pressure is exceeded. Even the Army has only recently begun trying to model bullet engraving for small arms in its software.

Thanks! Had a friendly disagreement on this same subject a couple of months back with a guy at the range. He supported his claim that pressure is increased with deeper seating by quoting QL calc's - I assumed at that point that QL did not consider bullet jump/distance to lands, but simply looked at case volume dependent on seating depth.

I can see where modeling bullet engraving would be a chore and include many variables!

It has to be more than a suspicion when you consider that the relative change in the volume of the powder chamber is but a small percentage of the capacity and volume, particularly in the larger cases.

Thanks guys, that's what I needed to know.

No offense JB, I almost always agree with you and I greatly respect your experience and knowledge, but I do believe you have missed this greatest contributor to pressure in many modern powders and rifles. Case capacity has a great and direct correlation to pressure, and seating depth directly impacts this affect. I have a Model 70 bolt action 284 Winchester and I can easily exceed most listed loads with zero pressure signs due to the fact that those listed loads minimize the oal to 2.800". I would challenge your conclusion based on this one cartridge, not to mention experience I have had with both a standard box Rem-700 7mm-08 and a Wyatt's box fitted 7mm-08. No comparison in what compressed loads can be used without any pressure signs. So in all due respect, I believe the gun that allows a longer Oal can always be loaded to a higher powder capacity and higher velocity. And if you review your answers to many similar questions, I believe you will find that your conclusions match this one. I have validated your very own 4 to 1 powder to velocity calcs with my Wyatt's 7-08, my 7-08ai, and my 284 win.
Respectfully,
bludog

bludog, if I understand you right, you are saying a rifle with a longer than standard throat can be loaded with add'l powder and achieve higher velocity? If so, I totally agree, but I think you're talking about something slightly different than this topic is discussing. We're not comparing one rifle with shorter throat to another with a longer throat which I believe is the example you're using, we're discussing same rifle, just seating deeper or further out. If I misunderstood you, I apologize.

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.