Reloading assumptions

[Marco Califo]

In other words, once the bullet passes that gap, it is vented, and pressure is decreasing.

[Aguila Blanca]

Originally Posted by RKG

Peak Chamber Pressure is the highest value that would appear on a graph of chamber pressure during bullet travel down the bore. Excessive Peak Chamber Pressure is what causes guns to blow up. Peak Chamber Pressure occurs at the point where propellant combustion ends, and depending on propellant speed, may be before the bullet begins to move or very shortly thereafter.

I must respectfully disagree with the bolded potion of the above statement.

I am primarily a handgun shooter, and ALL my reloading to date has been for handgun rounds. If you've ever seen the great balls of fire that come out of the muzzles of some handguns with some rounds, you would know beyond any doubt that the propellant combustion did not end when the bullet started moving down the barrel.

In fact, several years ago I started an experiment to quantify the loss of muzzle velocity shooting the same ammunition out of shorter barrels. I'm a 1911 guy at heart, so I gathered up a collection of Para-Ordnance pistols (I chose the same brand in the hope that barrel manufacturing would be similar enough to minimize the variable that I was NOT doing what Ballistics By The Inch does, and using a single test barrel that gets lopped off an inch at a time as they run each series of tests) with barrel lengths of 5", 4-1/4", 3-1/2", and 3".

I started out with the near screen of the chronograph ten feet from the muzzle. All went swimmingly for the 5" and 4-1/4" pistols, but the experiment unraveled when I started shooting the 3-1/2" P12.45. Out of ten shots, probably half registered "Error" rather than a velocity. It got even worse with the 3" P10.45.

I was testing five rounds each of several different commercial loads through each pistol, and it was the same ammo that caused the problem in most of the cases. It became clear pretty quickly that with the shorter barrels I was ejecting enough still-burning powder that the glowing particles were messing up the screens. (I was at an indoor range, using the powered infrared sky screens.)

Moving the chrony out to a distance of fifteen feet resolved the problem but, by the time I had that all figured out, I had used up the amount of time the range owner was willing to have that portion of the range shut down. I have always intended to run the experiment again, using two pistols in each barrel length rather than one, a fifteen foot chrony distance, and more rounds of fewer ammo types in order to get a better sampling. I just hanven't had the time to do it.

The point of all this is that if there's burning powder passing through the chronograph ten feet in front of the muzzle, the propellant clearly hadn't all stopped burning as soon as the bullet started to move. In fact, one of the keys to the so-called "short barrel" self-defense rounds most of the major ammo makers started offering about ten years ago is faster powders, with the goal being to have the pressure peak sooner rather than still be increasing when the bullet exits the muzzle.

[74A95]

Originally Posted by Aguila Blanca

In fact, one of the keys to the so-called "short barrel" self-defense rounds most of the major ammo makers started offering about ten years ago is faster powders, with the goal being to have the pressure peak sooner rather than still be increasing when the bullet exits the muzzle.

I hate to break the news to you, but pressure peaks way before the bullet has exited.

Even with a slow powder like 296, this is what QuickLOAD says about peak pressure and bullet travel:

357 Magnum, 158 grain Bullet, 16.0 grains of 296, maximum chamber pressure (Pmax) = 31,596 psi, bullet travel at Pmax = 0.43 inches.

Thus pressure peaks before the bullet has moved even half an inch. Pressure drops after the bullet moves past 0.43 inches and continues down the barrel. In a 4 inch barrel the gas pressure at the muzzle (according the QL) is 12,535 psi (this might not be a 'revolver' barrel).

[mehavey]

...the propellant clearly hadn't all stopped burning as soon as the bullet started to move.

As anyone who looks at the unburned powder kernels on the bench
ahead of the muzzle of mag cartridges burning slow powder.... knows.

As of course these guys found out the hard way about four years ago....
https://www.youtube.com/watch?v=2RkRTs9k7hY

[Aguila Blanca]

Originally Posted by 74A95

I hate to break the news to you, but pressure peaks way before the bullet has exited.

You're not breaking anything to me -- I know that. But that's not what the statement to which I responded said.

Originally Posted by RKG

Peak Chamber Pressure occurs at the point where propellant combustion ends, and depending on propellant speed, may be before the bullet begins to move or very shortly thereafter.

[RKG]

I don't want to get bogged down too far, or bore everyone to the point of taking up golf, but:

The flash often seen in some load situations does not mean that the propellant is still combusting after the bullet leaves the muzzle. What happens is that combustion generates a large mass of high pressure, high temperature, low oxygenated gas. As the bullet exits the muzzle, this mass of heretofore confined gas suddenly vents to a low pressure, low temperature, highly oxygenated atmosphere, and that causes some of this gas to oxygenate. That is not propellant still burning.

Yes, in some loads you will see granulates of unburned powder. That does not mean that propellant combustion was continuing as far as muzzle vent. It means that in some cases, some fraction of the propellant never combusts.

As for cylinder gap in revolvers, play close attention to the details: load, propellant, location of the test "cylinder gap." For instance, in a S&W .357 K-Frame, the nose of the bullet is almost at the same point as the front of the cylinder. Fire a .38 Special load of a 148 grain wadcutter with a load of 2.7 grains of Bullseye, and Peak Chamber Pressure does occur before the bullet begins to move. Fire a .357 load with a 158 grain jacketed slug pushed by 13-14 grains of H110/Win296, and Peak Chamber Pressure might well have occurred at the point of cylinder vent, but that is well short of about half an inch of bullet travel.

There are a lot of sources of literature on internal ballistics. Some are written in more pedestrian terms, though none are bedside reading.

[44 AMP]

Other than as an interesting intellectual exercise, and perhaps giving some insight into the kinds of things firearms designers, ammo makers and powder producers have to consider, I am wondering, just how knowing specifically where peak pressure occurs, is of any use to the average shooter.

Knowing what the peak pressure is, might be useful but I don't really see how. What I mean by that is, what use is knowing the actual number? What can you do with it, other than knowing it is below proof test levels?

There is a lot of good, tested, long established data to use as guidelines, which, if heeded, will keep reloaders well below actually dangerous pressures and in the range of "suitable" and "useful" as operating pressures for about everything there is, or that you could think up. Even wildcats follow the general rules covering powder capacity, burn rates and generated pressures.

If you aren't designing something, to be safe at various pressure levels, I think knowing the precise pressure numbers isn't useful for most of us, other than as one more subject to add to the discussion about how many angels can dance on the head of a pin.

As a friend of mine is fond of saying, "knowing that is interesting, but how does it put deer in my freezer using my Savage 99?..."

[Unclenick]

RKG,

Watch the video Mehavey linked to. Any time you sweep up a little pile of the dust out in front of the firing line at an indoor range (where it hasn't mixed with dirt or water), you will find you can light it with a match, and it will flare like powder because there is a lot of unburned powder in it. In the case of that video, apparently, whoever swept the floor had been sweeping it under the platform boards you see out in front of the line in the early part of the video and had been doing it for quite a while, as it would take quite a pile to flare that much.

What has occurred at the peak pressure point is not that the powder has burned out but rather that it has stopped generating gas fast enough to keep up with the rate of expansion behind the bullet scooting down the bore. In other words, the bullet's forward velocity is adding bore volume faster than new gas is evolving. This happens because most of the powder is burned and too little burning surface area is left to make gas fast enough, or, with progressive burning powders, it can just be because the majority of grains have exhausted the progressive portion of the burn (in a closed bomb vivacity test, this is the inflection point Z1 in the pressure curve). It should be noted that all grains don't ignite simultaneously, and particularly those furthest from the primer flash will start burning later and may still have a ways to go when the pressure starts to fall. You can sometimes find grains on the ground that don't appear to have started burning at all. This can happen particularly with full cases of slow rifle powders firing a lighter bullet (not an ideal combination), and the bullet actually creates such rapid expansion that grains at the front of the case, which blow forward together with the bullet, never finish having the flame front spread get to them.

For bullet movement, it has been noted by QuickLOAD's author, and with some independent testing, that magnum primers will sometimes produce lower and more erratic velocities in small powder space cartridges than standard primers do. This is the result of the primer pressure beginning to unseat the bullet before the powder burn gets fully underway, resulting in the starting volume for the powder burn being inconsistent from shot to shot. The 22 Hornet is famous for having this problem and needing to be loaded with pistol primers or other mild primers to avoid seeing much of it. My main point in mentioning this is just that the start of bullet movement can be hard to pin down.

We had a member many years ago who had worked for HP White laboratories. He said that for rifle cartridges, their equipment tests in the '70s had not detected bullets starting to move until pressures were at about 10,000 psi. This is right around where brass expands in earnest, and you can see from the way the mouths of sectioned rifle cases curl a little inward that the bullet was released when the neck expanded from shoulder to mouth. Once the mouth starts to leak gas, that expansion stops, as that leak causes a pressure gradient to form in the thin gap between the expanded neck and the bullet, dropping the pressure at the mouth nearer to that in the chamber around the bullet, leaving an inadequate pressure differential to further expand the mouth. The bullet will already have just started moving by then. QuickLOAD and GRT models have that starting at a somewhat lower pressure than HP White observed, being around 3600 lbs for jacketed bullets in rifle cartridges (lead slips in the neck more and pistol bullets have a lot of areas relative to the powder volume, so they move with less pressure), but the initial motion is in microinches, and the HP White equipment would have had difficultly detecting that.

The way you can tell where a bullet is at the peak pressure of a rifle is with a borescope. You look in to see where the copper fouling accumulation is greatest. When the pressure peaks, the bullet experiences maximum g force. That force tries to upset it outward, increasing bore friction and resulting in copper deposits. Usually, this is an inch or two down the bore.

[Nick_C_S]

Any time you sweep up a little pile of the dust out in front of the firing line . . . you will find you can light it with a match, and it will flare like powder because there is a lot of unburned powder in it.

I witnessed this ^ in real time at a range once. The person beside me was firing cap-n-ball. Evidently, a spark landed in front of the line and started a fire. The line had to go cold and the RSO had to put it out with an extinguisher.

It's a real thing.

[44 AMP]

When burnable powder comes out of the muzzle, for any reason, you can't say that is complete combustion.

[reynolds357]

Originally Posted by Aguila Blanca

I must respectfully disagree with the bolded potion of the above statement.

I am primarily a handgun shooter, and ALL my reloading to date has been for handgun rounds. If you've ever seen the great balls of fire that come out of the muzzles of some handguns with some rounds, you would know beyond any doubt that the propellant combustion did not end when the bullet started moving down the barrel.

In fact, several years ago I started an experiment to quantify the loss of muzzle velocity shooting the same ammunition out of shorter barrels. I'm a 1911 guy at heart, so I gathered up a collection of Para-Ordnance pistols (I chose the same brand in the hope that barrel manufacturing would be similar enough to minimize the variable that I was NOT doing what Ballistics By The Inch does, and using a single test barrel that gets lopped off an inch at a time as they run each series of tests) with barrel lengths of 5", 4-1/4", 3-1/2", and 3".

I started out with the near screen of the chronograph ten feet from the muzzle. All went swimmingly for the 5" and 4-1/4" pistols, but the experiment unraveled when I started shooting the 3-1/2" P12.45. Out of ten shots, probably half registered "Error" rather than a velocity. It got even worse with the 3" P10.45.

I was testing five rounds each of several different commercial loads through each pistol, and it was the same ammo that caused the problem in most of the cases. It became clear pretty quickly that with the shorter barrels I was ejecting enough still-burning powder that the glowing particles were messing up the screens. (I was at an indoor range, using the powered infrared sky screens.)

Moving the chrony out to a distance of fifteen feet resolved the problem but, by the time I had that all figured out, I had used up the amount of time the range owner was willing to have that portion of the range shut down. I have always intended to run the experiment again, using two pistols in each barrel length rather than one, a fifteen foot chrony distance, and more rounds of fewer ammo types in order to get a better sampling. I just hanven't had the time to do it.

The point of all this is that if there's burning powder passing through the chronograph ten feet in front of the muzzle, the propellant clearly hadn't all stopped burning as soon as the bullet started to move. In fact, one of the keys to the so-called "short barrel" self-defense rounds most of the major ammo makers started offering about ten years ago is faster powders, with the goal being to have the pressure peak sooner rather than still be increasing when the bullet exits the muzzle.

I would have to agree with you. No way peak pressure is reached when all the powder burns. All the powder does not burn until after the bullet exits the barrel. That is why we have muzzle flash.

[Unclenick]

Well, a lot of muzzle flash is also caused by unburned hydrogen coming out hot and igniting when it meets oxygen in the air. The role of flash hiders is to create a space that slows and cools expanding gases before they make full contact with the air, thus reducing combustion temperature and completeness.

Cellulose hexanitrate, the main form of nitrocellulose in smokeless powder, has what is called a negative oxygen balance of just over -24%, meaning there is a -24% shortage of enough oxygen in the molecule to combust all the hydrogen, carbon, and nitrogen it releases. So you still have some of those fuel molecules looking for a chance to find oxygen to burn them as they exit the muzzle. Hydrogen has the lowest ignition point, so if it is still hot enough, it is what burns first. Carbon has a higher ignition point in air and has a tendency to condense out on the cooler bore, which is part of why you get unburned carbon left over in the bore. 100% combustion wouldn't leave any carbon.

Nitroglycerin, on the other hand, has a positive oxygen balance of about 3½%, so it actually gives the nitrocellulose in a double-base powder a little oxygen help, but not enough to eliminate the majority of the unburned fuel gasses. Nonetheless, it provides a small but real increase in the energy that is extracted from the nitrocellulose, as well as providing its own energy, which is why double-base powders are often called "high energy" powders.

The bottom line is that you can get some flash whether the powder has burned out or not, particularly if the barrel is short. This was demonstrated to us during night firing at the Gunsite 270 basic rifle class. The difference in the flash from an 18" 308 bolt rifle barrel and from a 22" bolt rifle barrel using the same ammo and neither having a flash suppressor was dramatic. A big white fireball versus a much less voluminous yellow-orange flash. That difference is due to cooling as well as less burning powder remaining after going through the longer barrel.

[mehavey]

I love this place....

[reynolds357]

Originally Posted by Unclenick

Well, a lot of muzzle flash is also caused by unburned hydrogen coming out hot and igniting when it meets oxygen in the air. The role of flash hiders is to create a space that slows and cools expanding gases before they make full contact with the air, thus reducing combustion temperature and completeness.

Cellulose hexanitrate, the main form of nitrocellulose in smokeless powder, has what is called a negative oxygen balance of just over -24%, meaning there is a -24% shortage of enough oxygen in the molecule to combust all the hydrogen, carbon, and nitrogen it releases. So you still have some of those fuel molecules looking for a chance to find oxygen to burn them as they exit the muzzle. Hydrogen has the lowest ignition point, so if it is still hot enough, it is what burns first. Carbon has a higher ignition point in air and has a tendency to condense out on the cooler bore, which is part of why you get unburned carbon left over in the bore. 100% combustion wouldn't leave any carbon.

Nitroglycerin, on the other hand, has a positive oxygen balance of about 3½%, so it actually gives the nitrocellulose in a double-base powder a little oxygen help, but not enough to eliminate the majority of the unburned fuel gasses. Nonetheless, it provides a small but real increase in the energy that is extracted from the nitrocellulose, as well as providing its own energy, which is why double-base powders are often called "high energy" powders.

The bottom line is that you can get some flash whether the powder has burned out or not, particularly if the barrel is short. This was demonstrated to us during night firing at the Gunsite 270 basic rifle class. The difference in the flash from an 18" 308 bolt rifle barrel and from a 22" bolt rifle barrel using the same ammo and neither having a flash suppressor was dramatic. A big white fireball versus a much less voluminous yellow-orange flash. That difference is due to cooling as well as less burning powder remaining after going through the longer barrel.

So, when any component of powder does not burn in the barrel, the powder did not burn completely in the barrel.

[mehavey]

So, when any component of powder does not burn in the
barrel, the powder did not burn completely in the barrel.

No, Rather what was said is that muzzle flash is no less combusting gases, as unburned powder itself.

That actual powder is also left unburned is self-evident from the particulate residue on the bench/floor.

[higgite]

I don't believe muzzle flash is caused by unburnt powder, but I have seen sparks fly from the muzzle along with the muzzle flash which I attribute to partially burnt or unburnt powder being ignited by the muzzle flash that Unclenick described.

[Marco Califo]

Unburnt Powder, and hot gases making a fireball with fresh O2 outside the muzzle, are two very different things. If you want to see unburned powder try a compressed load of WC872 in anything. It will leave a trail down the barrel.
Hot gasses, generated by complete powder burn, will still produce muzzle flash as residual combustibles gasses hit fresh air. That does not mean the powder charge is still burning.
All the O2 required for combustion of powder come in the powders themselves. Otherwise you would need fresh air for firing, which you obviously do not.

[stagpanther]

Secondary pressure spike; something to consider though not everyone agrees.