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Old 09-02-2017, 09:01 PM
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Regarding Boyle's Law (P1V1=P2V2) where P is pressure and V is volume (of a fixed amount of gas), does anyone see why this can't be applied to reloading with respect to differences in case volume? For example, some people like to seat wadcutters to the crimp groove while others like to seat to the bullet completely inside the case with the crimp over the outside edge of the bullet. This is about a .15 inch difference in seating depth. So, the volume of a 38 special is about 23.7 gr with an internal case depth of .99 inches. After seating a wadcutter to a depth of .560" there is 10.29 gr H2O volume remaining with a pressure of 11,800 cup using 3.3 gr of Titegroup. If we seat the wadcutter completely to .575" with the same powder charge, we get a remaining case volume of 9.93 gr. Applying Boyle's law, we get a pressure of 12,227 cup.

Since the same powder charge should produce the same amount of gas in either load, I see no reason that Boyle's Law couldn't be used to estimate pressure when the remaining case volume is the only variable that changes.

I know there are others that know a great deal more about reloading and gas laws than me, and I'd like to get some opinions about whether or not this might produce a fairly accurate estimate of pressure.
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Old 09-02-2017, 09:20 PM
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Boyle's law as you stated it, applies 1) where temperature is constant, and 2) at a particular moment in time; it is not dynamic.

With most smokeless propellants, the rate of conversion from powder to gas increases with both temperature and pressure. So, in a smaller space, temperature and pressure build more quickly which accelerates conversion at the time when there is more unconverted powder still in the case.

In the example using 38 Special, the pressures you derived seem very close to published figures, but note that in the Speer Manual #11 (and many subseqent editions) there is a report in the section on 9mm about making sure there is sufficient neck tension to prevent bullet set-back because it can create dangerous pressure noting that bullets intentionally seated just 0.030 inch deeper nearly doubled in pressure and I don't think that follows a static Boyle's law analysis.

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Old 09-02-2017, 11:41 PM
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Intrigued, I ran a few Quickload simulations using loads and powders I have used. The first few QL runs showed very similar results with QL's prediction being about 8% higher peak pressure than Boyles law. Here's one such example (UCV=Usable Case Volume in H2O grains):

9mm, 115gr Rainer RN, HP38 4.7gr:
-- COL=1.169, Pmax=24781, UCV= 10.027
-- COL=1.100, Pmax=32440, UCV= 8.301
-- Boyles Law = 24781x10.027/8.301=29933

So "actual" pressure in this and other examples increased ~8% more than Boyles Law predicted.

Then I decided to change powders, and got a distinctly different result:

9mm, 115gr Rainer RN, TiteGroup 4.7gr:
-- COL=1.169, Pmax=19479, UCV= 10.027
-- COL=1.100, Pmax=41486, UCV= 8.301
-- Boyles Law= 19479*10.027/8.301=29529

QL's predicted Pmax is 76.3% higher than Boyle's law would project. (Be carefuller with TiteGroup lol).

So as hdwhit said . . . there's other stuff going on here.

Last edited by Twoboxer; 09-02-2017 at 11:43 PM.
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Old 09-03-2017, 12:01 AM
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Boyle's Law assumes the gas is in a state of equilibrium. The correct equation is PVT, where T is the absolute temperature (Kelvin). It also assumes ideal gas behavior, which requires no interaction between gases in a mixture. Ignition in a firearm meets none of these criteria.

The critical factor for reloading is peak pressure, which is dependent on the rate of conflagration. That rate is strongly dependent on pressure, temperature, and characteristics of the powder, including composition, shape and surface area (which changes in time). A flake powder is likely to burn faster than extruded powder, and extruded powders often have perforations to increase the initial rate of burning, and maintaining that rate since the surface area decreases as the rate of burning increases.

It is fairly straight forward to express this process with differential equations, but those equations can't be solved - there is no explicit solution. If you wonder why scientists can't predict the weather, it is precisely for the same reason. The equations can't be solved without assumptions and gross simplification. Astronomers can't predict the next asteroid to destroy life on earth (or even streak across the sky) because there is no solution to 3 body gravitation. (How, then, is Global Warming "settled science"?)

That said, the best way to predict the pressure is to measure it, many times under controlled conditions, to produce an estimate with an estimated margin of error, then load on the safe side of that estimate.

Last edited by Neumann; 09-03-2017 at 12:07 AM.
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Old 09-03-2017, 12:12 AM
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Even black powder will detonate, producing pressure in excess of 100,000 psi, if compressed to a point that there is less than about 10% free space. It will also detonate if not compressed about 10% from its free flowing volume. If you get in a hurry, that's the difference between a muzzle loader and a pipe bomb (per Hickok45).

Some smokeless powders can produce dangerous pressure spikes if loaded lighter than recommended.

Anyone feeling lucky?

Last edited by Neumann; 09-03-2017 at 12:14 AM.
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Old 09-03-2017, 10:42 AM
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Actually the correct formula for Ideal Gas Law is PV=NRT and it is derived from Boyle's Law. P=Pressure, V=Volume, N=the Total Mass of the Gas, T=Temperature in degrees Kelvin, and R=the Ideal Gas Constant.

Now, if you use the Ideal Gas Law factors such as the Rate of Expansion for a gas can be factored in as can the Change in Volume during an event. While the math involved goes beyond simple Algebra into Differential Equations it can be used to predict the pressure at a particular instant in Time, which is exactly how programs such as Quickload work. For example give a real whiz at Differential Equations the Rate of Expansion for a particular powder (a more accurate description of what we commonly call Burn Rate), the diameter of the bullet and it's mass, and a specific instant of time after ignition and he could sit down with a pencil, paper, and simple slide rule and give you a fairly close estimate of the pressure.

So, I wouldn't say that Boyle's Law doesn't apply because it is actually at the core of a complete description of Internal Ballistics. However it is too basic for this task and in order to get to the correct solution you have to use the Ideal Gas Law and Applied Mathematics to get there.
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Old 09-03-2017, 11:23 AM
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Unfortunately I have no method of measuring direct pressure or temperature of the expanding gas. I was hoping this would prove to be a good (or at least moderately acceptable) method to estimate pressures of .38 LC from .38 special load data. As of late, I've been loading MBC Cowboy #2 .38 spl with AutoComp, and I like the results. I'd like to load some .38 LCs with it, but I can't find any load data for that particular cartridge. I figure that the min AutoComp load for .38 special (7300 PSI/7.7 inch barrel) would be plenty low in a LC case, but I'd just like know approximately what the pressure would be in the LC case. Boyle's Law gives about 9300 PSI.
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Old 09-03-2017, 11:31 AM
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Wow.
I enjoyed reading all the explanations. Good to see all that "education" out there with re-loading!
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Old 09-03-2017, 12:02 PM
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Quote:
Originally Posted by Neumann View Post
Boyle's Law assumes the gas is in a state of equilibrium. The correct equation is PVT, where T is the absolute temperature (Kelvin). It also assumes ideal gas behavior, which requires no interaction between gases in a mixture. Ignition in a firearm meets none of these criteria.

The critical factor for reloading is peak pressure, which is dependent on the rate of conflagration. That rate is strongly dependent on pressure, temperature, and characteristics of the powder, including composition, shape and surface area (which changes in time). A flake powder is likely to burn faster than extruded powder, and extruded powders often have perforations to increase the initial rate of burning, and maintaining that rate since the surface area decreases as the rate of burning increases.

It is fairly straight forward to express this process with differential equations, but those equations can't be solved - there is no explicit solution. If you wonder why scientists can't predict the weather, it is precisely for the same reason. The equations can't be solved without assumptions and gross simplification. Astronomers can't predict the next asteroid to destroy life on earth (or even streak across the sky) because there is no solution to 3 body gravitation. (How, then, is Global Warming "settled science"?)

That said, the best way to predict the pressure is to measure it, many times under controlled conditions, to produce an estimate with an estimated margin of error, then load on the safe side of that estimate.
I wish I could give 'Newmann's' post two or three likes! I was reading and wondering why someone doesn't mention the huge number of variables in internal ballistics. A Prof in a college Construction Estimating once told my class to "be very careful in making assumptions while working to an estimate." He went on to say that one assumption can be workable, two is getting on the edge of garbage results and three assumptions assures the wrong result.

In my construction management career, I used those words of wisdom more than once. Empirical Observation and judicious use of Statistics will lead to solid guidance in reloading results. When I was No. 2 Gunsmith at the custom rifle shop in Reno, I worked with a graduate Physics Major who was the front retail clerk. I was the shop's designated reloader of wildcat rifle cartridges for our house built rifles. He and I would pour over reloading manuals trying to interpolate loading combinations. He would fill page after page of paper with calculations trying to predict pressure and external ballistics. We had a range that went to 500 yds. He would predict 'drop' at five hundred yards with a 100 yd. zero. I was also the designated test shooter of all the custom rifles and custom cartridge loads. My physics major friend would give me the predicted 500 yd. drop of a load before I went to the range. I would get the 100 yd. zero right on and then after the rifle cooled shoot a three shot group at 500 yds. on a 4'x8' plywood standing upright with a black bullseye about 12" from the top. I don't ever remember the group hitting exactly where the physics major predicted. There are just too many variables.

By the time I started my Professional Gunsmithing efforts, I knew full well that every firearm/cartridge load were a law unto itself to some extent. It was part of my job to find out to what degree that 'extent' was.
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Old 09-03-2017, 02:25 PM
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Originally Posted by Mr_Flintstone View Post
Unfortunately I have no method of measuring direct pressure or temperature of the expanding gas. I was hoping this would prove to be a good (or at least moderately acceptable) method to estimate pressures of .38 LC from .38 special load data. As of late, I've been loading MBC Cowboy #2 .38 spl with AutoComp, and I like the results. I'd like to load some .38 LCs with it, but I can't find any load data for that particular cartridge. I figure that the min AutoComp load for .38 special (7300 PSI/7.7 inch barrel) would be plenty low in a LC case, but I'd just like know approximately what the pressure would be in the LC case. Boyle's Law gives about 9300 PSI.
A .38 SHORT colt case is just the tiniest bit LONGER than a 9x19 case. I have measured H20 VOLUME in both (new Starline cases) the SC holds a tiny bit more. I load .358 lead 125 grains in same with same charge. (Now these are not near max charges for 9x19, I am looking for efficient burns in 1.75 to 2.25 inch barrels. NOTICE I AM NOT PROVIDING ANY DATA.) I cannot imagine that the pressure in the tiny bit bigger case would be more than in the 9x19 and I have published data for that. And a .38 LC is only 1/3 as much smaller than .38 special as .38 SC. I don't assume I can work lower volume higher pressure safely, I do assume I can work higher volume lower pressure safely. Note I am not saying the pressure in the .38 SC would be ok in any .38 special handgun. BUT any published 9x19 load under .38+p 20k limit should be ok in a +p gun assuming same bullet and same depth, PLUS .357 has a higher pressure limit than 9x19 AND some .38s have interchangable 9x19 cylinders. Obviously the frame and barrel are ok with 9x19 pressure. I tested each load in a k frame .357 before use in a 640. I am not saying you should do it, but check other forums, its popular with people who shoot speed matches, oh like Jerry Miculek.
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Old 09-15-2017, 09:42 AM
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Quote:
Originally Posted by Mr_Flintstone View Post
Regarding Boyle's Law (P1V1=P2V2) where P is pressure and V is volume (of a fixed amount of gas), does anyone see why this can't be applied to reloading with respect to differences in case volume? For example, some people like to seat wadcutters to the crimp groove while others like to seat to the bullet completely inside the case with the crimp over the outside edge of the bullet. This is about a .15 inch difference in seating depth. So, the volume of a 38 special is about 23.7 gr with an internal case depth of .99 inches. After seating a wadcutter to a depth of .560" there is 10.29 gr H2O volume remaining with a pressure of 11,800 cup using 3.3 gr of Titegroup. If we seat the wadcutter completely to .575" with the same powder charge, we get a remaining case volume of 9.93 gr. Applying Boyle's law, we get a pressure of 12,227 cup.

Since the same powder charge should produce the same amount of gas in either load, I see no reason that Boyle's Law couldn't be used to estimate pressure when the remaining case volume is the only variable that changes.

I know there are others that know a great deal more about reloading and gas laws than me, and I'd like to get some opinions about whether or not this might produce a fairly accurate estimate of pressure.
If the bullet entered free space after leaving the cartridge case you could make a pretty good relative determination of case pressure by using Boyle's law but the bullet enters the bore and that is where the majority of the powder is burned. Therefore the burn rate of the powder, barrel length, bore diameter, bullet friction, expansion ratio etc have much more of an effect on peak chamber pressure than small changes in case volume it would seem to me...
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Old 09-15-2017, 11:29 AM
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Quote:
Originally Posted by kingrj View Post
If the bullet entered free space after leaving the cartridge case you could make a pretty good relative determination of case pressure by using Boyle's law but the bullet enters the bore and that is where the majority of the powder is burned. Therefore the burn rate of the powder, barrel length, bore diameter, bullet friction, expansion ratio etc have much more of an effect on peak chamber pressure than small changes in case volume it would seem to me...
In most cases peak pressure is reached before the bullet leaves the brass.

be safe
Ruggy
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Old 09-15-2017, 09:54 PM
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After reading all this,I've just understood,almost 45 years too late why I slumped Physics and Chemistry at school!
Qc
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Old 09-15-2017, 10:35 PM
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Gay-Lussac's law is far more appropriate in any discussion of pressure generated inside a gun barrel than Boyle's Law. G-L's law says that pressure is directly proportional to temperature (absolute temperature) in a closed system. When smokeless powder burns, it not only produces gaseous products of combustion, it also heats those gasses to a very high temperature, which produces a correspondingly high pressure. The more energy contained in the propellant, the higher the pressure. Single-base propellants basically contain only nitrocellulose (NC). Double-base propellants contain both nitrocellulose and nitroglycerine (NG), NG contains considerably more energy per unit of mass than NC, therefore double-base propellants are more energetic and produce higher temperatures and greater pressures. There is much more detail needed to completely define the thermodynamics, but that's what the whole science of interior ballistics is all about. So I'll stop here.
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Old 09-16-2017, 09:47 AM
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In most cases peak pressure is reached before the bullet leaves the brass.

be safe
Ruggy
Would not that depend on the powder used? I can understand Bullseye maybe doing that but 2400 or 296???
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Old 09-16-2017, 12:32 PM
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Would not that depend on the powder used? I can understand Bullseye maybe doing that but 2400 or 296???
Of course it does.

Also the quantity of that powder.

Having chonographed loads in several .357 remington magnum handguns identical except for barrel length I will say that (not really a surprise here) 3 grains of BE burns in the case. 15 grains of 2400 burns in the case in the bore and depending on the length of the barrel otside the muzzle.

My idea of fun is finding optimal loads for specific guns. With experience you get a real feel for the dynamics involved.
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Old 09-16-2017, 01:05 PM
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If you are working with a reloading manual, using simple arithmetic and proportions, are at or near the middle of the load data values, are using reasonable equivalent bullets; you can reasonably determine a safe starting load. If it takes physics, inorganic chemistry, mass transfer theory, differential equations, or inexplicable laws of science; you should perhaps stop before you hurt your self or your gun.

Experimentation can be done without a laboratory full of measuring instruments or main frame computer. The key is common sense. The differences between loadings for 44 Russian, 44 Spl, and 44 Mag are of not much value when determining a safe load for a 35 caliber cartridge. Ultimately, close enough NOT maximum load works. If you feel the starting load is 4.0 to 4.3 grains, use 4.0 grains of powder for 5 rounds.

The Science of Reloading is a very interesting subject. The first step is start with a wheelbarrow load of assumptions that unfortunately may be: Wrong, Inaccurate, Not Applicable, or You Really Made a Big Mistake.

I have multiple semesters of Math, Physics, Thermodynamics, and Fluid Mechanics to finally get my Mechanical Engineering degree. I try not to use anything more than simple arithmetic and proportion when reloading.
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Old 09-16-2017, 01:39 PM
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I just learned that I use algebra to convert grains to volume. I never understood algebra and still don't. at least I think it's algebra.
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Old 09-16-2017, 03:06 PM
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Would not that depend on the powder used? I can understand Bullseye maybe doing that but 2400 or 296???
Yes the type of powder influences peak pressure but so do other values.

With that said, I have never seen a straight wall pistol cartridge where peak pressure was not reached before the bullet left the brass regardless of powder used.

The quantity of powder in most instance moves the peak pressure point very little in retaliative terms, though as would be expect more powder results in raised the pressure every thing else being equal.

Remember peak pressure and mean pressure are two very different things.

A chronograph tells you very little about peak pressure but does provide some indication of the mean pressure.

Powder burning outside the barrel does not has no bearing on the peak pressure or were it occurs.

good luck and be safe
Ruggy
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Old 09-17-2017, 07:06 AM
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Quote:
Originally Posted by ruggyh View Post
Yes the type of powder influences peak pressure but so do other values.

With that said, I have never seen a straight wall pistol cartridge where peak pressure was not reached before the bullet left the brass regardless of powder used.

The quantity of powder in most instance moves the peak pressure point very little in retaliative terms, though as would be expect more powder results in raised the pressure every thing else being equal.

Remember peak pressure and mean pressure are two very different things.

A chronograph tells you very little about peak pressure but does provide some indication of the mean pressure.

Powder burning outside the barrel does not has no bearing on the peak pressure or were it occurs.

good luck and be safe
Ruggy
Very interesting indeed! I have seen some time/pressure curves for rifles and the peak pressures seem to be about 2-3" down the bore so with faster handgun powders it would make sense that it occurs closer to the chamber but I have not been able to find any specific curves for handguns. I do realize that once the bullet leaves the barrel the pressure essentially goes to zero. Since you have apparently seen handgun data or may have even developed some of this test data I would appreciate it if you could point to where I can find some of it. Thanks!
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Old 09-17-2017, 07:44 AM
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All that math makes my head hurt!!!

I'm more of a point and click/pictures worth a 1000 words reloader.

Back in the day the 38spl/wc/bullseye combo was the bad boy on the block. A lot of kabooms were happening so alliant put this out trying to inform reloads of the common mistakes being made.


Quick loads is an excellent program that shows the difference in pressures/bullet seating depth. The 9mm/lee 120gr cast bullet is a popular combo. Unique is good powder for the 9mm that very forgiving/no huge spikes in short start pressures.


Ramshot powder company put this out in their reloading manual. There's a lot of kabooms happening with the 9mm & 40s&w's and fast burning (high pressure spiking powders) powders. A graph with ramshot zip and the affects of different seating depths.


Perhaps you could use your Boils law and compare your findings to what the mfg's and reloading software makers found.
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Old 09-17-2017, 12:41 PM
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It's a little difficult to show without some graphs, but the only way peak pressure could occur before the bullet moves is if the propellant burned instantaneously. And that does not happen. The peak and decline pressure curve shape depends upon the relative quickness of the propellant. Assuming identical muzzle velocities, the pressure from a load with a propellant of high relative quickness will peak early and decline rapidly. The pressure curve from a propellant having a lower relative quickness will peak later (and lower) and decline more slowly.

Those who understand calculus may be interested to know that so long as the bullet weight and muzzle velocity are constant, the area under the pressure-time curve (its integral) must be the same for propellants having differing relative quickness. But the curve shapes will be different.

Last edited by DWalt; 09-18-2017 at 05:28 PM.
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