Pressure & OAL
- Thread starter Cola308
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In high power rifle, jacketed bullet can raise peak pressure about 20% by making contact with the lands. However, a good portion of that increase is thought to be due to the contact cutting off the bypass of evolving gases that slow the rise in pressure. A rifle cartridge is usually loaded with slower progressive burning powder that peaks when the bullet is an inch or two into the bore. If it start burning faster, then the pressure peaks a little sooner when the bullet has not traveled as far and therefore has a smaller total volume behind it.
On the flip side, if you seat a bullet too deeply, that also raises pressure because of the base of the bullet using up powder space, again reducing the volume the powder starts producing gas in and so, again causing the rise in pressure to speed up. So a high power rifle bullet usually has a high spot in pressure touching the throat that drops about 20% as you push the bullet in a bit deeper and then has a trough in curve, after which seating still deeper sees pressure start to climb again. 0.030" or so off the lands is plenty for avoiding the peak at the jammed-in end with most bullets, but it does vary with the bullet profile shape, and a bullet whose ogive has a shallow angle along its sides may have to go still deeper for pressure to bottom out.
A pistol is a bit of a different animal. The powder is faster so the gas bypass mechanism doesn't hold down its pressure so much. In the .45 Auto I've intentionally shot lead bullets seated out to touch the lands for years. It improves accuracy and reduces leading. Doing that with jacketed bullets doesn't seem to improve accuracy or fouling any, so I don't bother, giving the bullets a little wiggle room for easier feeding.
But seating too deep is another matter. Most pistol rounds have tight powder spaces and the bullet it as wide as the case. Both factors mean the bullet doesn't have to be seated very much too far to take up a big percentage of the powder space, raising pressure. The Accurate manual has a plot of pressure increase with seating depth in the 9 mm that is a bit alarming. So the best advice is to ask the bullet maker what the COL should be for the particular bullet you are loading and start there. The length number usually given in a loading manual is the SAAMI maximum that is supposed to guarantee fit in standard magazines, though the number is usually too long for pistol bullets with very wide blunt or flat noses. There, again, consult the bullet maker.
On the flip side, if you seat a bullet too deeply, that also raises pressure because of the base of the bullet using up powder space, again reducing the volume the powder starts producing gas in and so, again causing the rise in pressure to speed up. So a high power rifle bullet usually has a high spot in pressure touching the throat that drops about 20% as you push the bullet in a bit deeper and then has a trough in curve, after which seating still deeper sees pressure start to climb again. 0.030" or so off the lands is plenty for avoiding the peak at the jammed-in end with most bullets, but it does vary with the bullet profile shape, and a bullet whose ogive has a shallow angle along its sides may have to go still deeper for pressure to bottom out.
A pistol is a bit of a different animal. The powder is faster so the gas bypass mechanism doesn't hold down its pressure so much. In the .45 Auto I've intentionally shot lead bullets seated out to touch the lands for years. It improves accuracy and reduces leading. Doing that with jacketed bullets doesn't seem to improve accuracy or fouling any, so I don't bother, giving the bullets a little wiggle room for easier feeding.
But seating too deep is another matter. Most pistol rounds have tight powder spaces and the bullet it as wide as the case. Both factors mean the bullet doesn't have to be seated very much too far to take up a big percentage of the powder space, raising pressure. The Accurate manual has a plot of pressure increase with seating depth in the 9 mm that is a bit alarming. So the best advice is to ask the bullet maker what the COL should be for the particular bullet you are loading and start there. The length number usually given in a loading manual is the SAAMI maximum that is supposed to guarantee fit in standard magazines, though the number is usually too long for pistol bullets with very wide blunt or flat noses. There, again, consult the bullet maker.
As noted, rifle and pistol are a little different because of the space available in the case.
Hornady describes the rifle pressure here: https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics
A plot of pressure and OAL in the pistol can be found on page 3 of this document: http://www.castpics.net/LoadData/Freebies/RM/Ramshot/Ramshot_3.pdf
Hornady describes the rifle pressure here: https://www.hornady.com/team-hornady/ballistic-calculators/ballistic-resources/internal-ballistics
A plot of pressure and OAL in the pistol can be found on page 3 of this document: http://www.castpics.net/LoadData/Freebies/RM/Ramshot/Ramshot_3.pdf
Seating depth is important as seating a bullet deeper will increase pressure. However, if someone is concerned about pressures and wants a verified and perfectly repeatable way to decrease pressures, cut your loads.
The amount of powder in the case is directly proportional to the amount of pressure created during powder ignition. This simple fact is often ignored by reloaders who often attempt to reduce pressure indications by changing reloading equipment, such as sizing dies, or changing primers or cases. The absolute first thing to do when confronted with evidence of excessive pressures is to reduce the amount of powder in the case.
The amount of powder in the case is directly proportional to the amount of pressure created during powder ignition. This simple fact is often ignored by reloaders who often attempt to reduce pressure indications by changing reloading equipment, such as sizing dies, or changing primers or cases. The absolute first thing to do when confronted with evidence of excessive pressures is to reduce the amount of powder in the case.
Did you intend to say that?
While it is true the pressure produced by a particular powder in a particular cartridge does increase with increases in the amount of powder, the relationship is NOT linear.
Consider this from the Hodgdon Manual, 25th Edition (April 1987) for .223 Remington:
-- 50 grain bullet, BL-C(2), 26.0 grains, 34,200 CUP
-- 50 grain bullet, BL-C(2), 28.8 grains, 47,100 CUP
In this example, a 2 grain (i.e. 7.7%) increase in powder charge results in an increase in pressure of 12,900 CUP (i.e. 37.7%), clearly not a "directly proportional" relationship.
In general, increasing OAL decreases chamber pressure, but as Unclenick already pointed out, if you increase OAL long enough that the bullet is effectively held in place by the start of the rifling then it can actually increase chamber pressure (because the pressure has to rise high enough to simultaneously move the bullet and start swaging it into the rifling).
Knowing whether seating a bullet a little long is liable to increase (rather than decrease) pressure is something that comes with long experience (unless you make the investment in chamber pressure measurement equipment), so the best advice is to stick close to the published OALs.
yellowfang
Inactive
With all due respect, you've provided a two-point data set, so we can't say whether it's linear or not.
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I don't think we need to show any more data until the poster explains a little more in detail what they meant . If you look at the quote i provided . The poster bid not say "a" powder or anything else specific . Only that powder in a case is directly proportional. In both our examples above i believe we have shown that is not accurate as far as i understood that quote to read .
Maybe and maybe not. I have had one instance of over pressure that was resolved simply by seating the bullet a little deeper which in effect increased bullet jump which in effect reduced pressure.
In bottle necked rifle cartridges, increasing bullet jump reduces pressure. Seating a bullet deeper in this reference WILL reduce pressures. All this within reason of course.
I am not condoning this method if you blow a primer for example. You have to investigate the circumstances and if you don't have the capabilities to do so, then it is your responsibility to make changes as suggested in order to stay out of the emergency room.
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I am the fan of the running start, I want my bullets to have 'that jump'. There is nothing entertaining about wondering if the bullet is going to get past the rifling before the pressure gets serious. I want my bullet past the rifling before the bullet knows the rifling is there.
Back to the shooter that volunteered to zero a rifle for a friend. By the time he was finished he had decided to sue all involved. He took the rifle to a North Texas gunsmith for repairs. The shooter had purchased a box of 308 W for a 25/06 rifle. As always at a gun shop there are kibitzers, most of them thought the bullet had to be 3" long when it left the barrel
F. Guffey
Back to the shooter that volunteered to zero a rifle for a friend. By the time he was finished he had decided to sue all involved. He took the rifle to a North Texas gunsmith for repairs. The shooter had purchased a box of 308 W for a 25/06 rifle. As always at a gun shop there are kibitzers, most of them thought the bullet had to be 3" long when it left the barrel
F. Guffey
You are right. I have been misusing the term directly proportional. Directly proportional https://en.wikipedia.org/wiki/Proportionality_(mathematics) according to Wiki means a linear function. The slope of the pressure curve is exponential, it in not linear.
I would like someone to put forth a better statement that is factual, precise and concise about powder chargers and pressure. That is, what is a better way of saying that the amount of powder in the case is XXXX to the amount of pressure in the case. Leading to the conclusion of, you want less pressure, use less powder.
For me, that is the first thing to do and the simplest method of achieving less pressure.
I am not fond of statements such as this. If you seat to 2.80" or 2.81" you should know the definite clearance. You get close enough, and bullet tolerances can put you in the danger zone.
I will not ever recommend anyone seating to the lands unless they have lots of experience and the sad thing is the one pulling the handle is the only judge of that.
I won't tell anyone what to do, but I will say what not to do.
Thus, if you do not know your chambers measurements, then stick with the book values. Most times the book knows way more than you do. Comprende??
I do know the clearance. These rounds we're not loaded by me but I measured them and they are 2.80 I know what bullet and powder used and checked my manual to see what it said IE. 2.70 but at 2.80 they are not touching the lands in my rifle. If they are not safe to fire in my rifle I will break them down or seat them at 2.70. Just looking for input to make a decision on.