Do bottleneck cartridges increase pressure?

[dgang]

Wanted to ask a question that puzzles me. Do bottleneck cartridges increase pressure? I can load 22gr. of H110 under a a110gr. bullet and achieve about 35,000 PSI. However, if I load 22gr. of H322 in a .223 case under a 69gr. bullet I'm loading about 55,000 PSI. Why does a smaller case (.357) with a much faster powder and a heavier bullet result in less pressure? I was told that a bottleneck cartridge does not increase pressure. What goes?
Thanks in advance and good shooting' to ya, Dgang

[std7mag]

I'm thinking different firearm designs.
And you listed 2 different bullet weights, with 2 different powders.

Biggest thing going for bottle neck cartridges is volume.
You are adding more space for powder for a given bullet diameter.

Think engines, the 4 cylinder will get you 100 hp. The V8 will get you 300.
Just very generic.

[F. Guffey]

Do bottleneck cartridges increase pressure?

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Wanted to ask a question that puzzles me. Do bottleneck cartridges increase pressure? I can load 22gr. of H110 under a a110gr. bullet and achieve about 35,000 PSI. However, if I load 22gr. of H322 in a .223 case under a 69gr. bullet I'm loading about 55,000 PSI. Why does a smaller case (.357) with a much faster powder and a heavier bullet result in less pressure? I was told that a bottleneck cartridge does not increase pressure. What goes?
Thanks in advance and good shooting' to ya, Dgang

Somewhere in there has to be something said about changing one component at a time. I can load a 30/06 bottle neck case with 46 grains of H4865 with a 150 grain bullet with lots of room between the top of the column of powder and base of the bullet. To raise pressure all I have to do is seat the bullet deeper to raise pressure.

OR; I can change powder to H1000 with the same case and same bullet and have all kinds of trouble getting the bullet to seat without crushing the powder.

And in the old days the devil may care old reloaders did not measure by weight and or volume, they just dipped the case into the powder to fill it up and then seat the bullet. And then they would say "You just can not get enough powder in the case when using this slow military powder".

I avoided the powder and the reloader that was that irresponsible.

F. Guffey

[HiBC]

What I'm going to tell you is not a perfect answer.It may help you get your head around your question.
The term that applies is "expansion ratio".
The issue is less about the case shape and more about the volume of the bore as a ratio to the volume of the combustion chamber.
As the bullet moves out of the case neck and into the bore,the volume of the bore behind the bullet is added to the volume of the combustion chamber.


As the powder burns,it produces a volume of gas. Pressure increase is about volume of gas produced vs volume available to occupy.


With a cartridge like the 30 carbine,to double the combustion chamber volume the bullet only has to move about an inch and a half.


Picture a cartridge like the 25-06. The area of the bullet base is far less,so the volume increase per inch of bullet travel is far less. But the larger case volume holds more powder,which is rapidly doubling,quadrupling,etc in volume.


So there is a race between the volume of powder gas being produced and the increase of available volume behind the bullet.


Play with the math. Use the formula "pi times radius squared= area of a circle.

[5whiskey]

F. Guffey is on it. 22 grains of one type of powder does not equal 22 grains of a different powder, and burn rate has nothing to do with it... density does. Take Hodgdon's .308 loading table. For a 155 grain bullet, 51 grains of H414 is the max load, while only 46 grains of H4895 is a max load. H414 burns much slower than H4895, so what gives? Energy per grain of powder has little to do with burn rate of the powder (although looking at load data and burn rate charts, generally slower powders will have heavier charge weights. Generally, not a rule but a guide).

And as far as the original question, a ratio of bore diameter to case capacity does in fact have an affect on pressure. Look at a 150gn projectile in 30-06 in a loading manual, and compare it to the same projectile in .270. The cases have nearly identical capacity (slight difference, but not very much), yet .270 max charges are MUCH lower than the max for .30-06 cases are with given powders. Bore diameter affects pressure if the powder charge weight and projectile weight is constant.

In light of this, it could be concluded that bottlenecking a cartridge does increase pressure if the powder/charge weight/projectile weight are constant. Lot's of variables involved in pressure to consider however.

[Metal god]

and burn rate has nothing to do with it... density does. Take Hodgdon's .308 loading table. For a 155 grain bullet, 51 grains of H414 is the max load, while only 46 grains of H4895 is a max load. H414 burns much slower than H4895, so what gives?

I disagree , What gives ? the burn rate . Why can't you put 40gr of Titegroup in that 308 case ? Burn rate - OK it's density then, hmm Why can't you put 40gr of Unique in that 308 case ? 1 it won't fit and 2 burn rate . You see this in just about every cartridge . The faster the burn rate the lower the charges will be to get the same velocities up until you blow your gun up with the faster powder trying to get velocities you can only get with a slower powder . Density may play a roll but burn rate is a much larger factor or my understanding of internal ballistics is just flat out wrong .

[hounddawg]

and once again a thread is completely derailed. Ninth grade physics tells me that anytime a flow of gas is restricted the pressure should increase

https://www.differencebetween.com/di...e-and-vs-flow/

now since I know someone will run right back in and derail with a gibberish post here is some real information on powder density and burn rates

https://www.differencebetween.com/di...e-and-vs-flow/

[HiBC]

I could be wrong,but for a given weight of nitrocellulose you will probably produce a certain volume of gas.

In my post,I mentioned a race between the gas expanding and the combustion chamber volume behind the bullet expanding.

Certainly burn rate is a factor in how rapidly the gas is produced.That IS one factor in the "race"

There are a number of variables involved in pressures,burn rate being an importabt one.

The OP was about bottleneck cases and how does bottleneck influence pressure.Comparing Bullseye to H-1000 will certainly produce dramatic result,but it has nothing to do with the influence of the bottleneck case. Another trip off into the weeds

IMO, the bottleneck itself is mostly a pathway to large case volume/small bore. IMO,expansion ratio is the factor to observe,more than case form.

Houndawg,I get your point about restriction . No argument that in a free flowing condition,a smaller orifice will produce more back pressure.


Does that work the same when a bullet is providing the back pressure?

[MarkCO]

Burn rate does have something to do with it, so does form factor (bottleneck v. straight-walled), energy density and expansion ratio. The contribution of each to the attained velocity of the ejecta is, in essence, partially dependant on the other variables. The primary variables are the mass of the charge and the burn rate and those two will predict the contribution of the other variables. It is neither an adiabatic or isothermal process, but small segments can be modeled as such and then added together. (And you all thought calculus was worthless)

Firing a cartridge in a gun is basically modeled, thermodynamically, as a heat engine where the case head to the base of the bullet is the combustion chamber and the bullet is a free piston. Conversion efficiency is largely based on the form factor (case volume to bore diameter and length). 30-40% of the energy is converted to piston (bullet) motion in the bore while gas expansion, friction and heating account for most of the rest. Yes, there is leakage and unburned powder as well as some engraving losses.

So, in answer to the OPs question. If we start with a high pressure rifle round, yes, the bottleneck does result in a pressure increase, and an efficiency increase as compared to a theoretical comparison to equal case and bore volume without any necking and the same ejecta mass. Conversely, if you start with a low pressure pistol round as compared to a theoretical equal case and bore volume with a necked case and the same ejecta mass, there will likely be no measurable contribution from the form factor.

[hounddawg]

you can check volume vs density on the Lee dipper chart

[5whiskey]

I disagree , What gives ? the burn rate . Why can't you put 40gr of Titegroup in that 308 case ? Burn rate - OK it's density then, hmm Why can't you put 40gr of Unique in that 308 case ? 1 it won't fit and 2 burn rate . You see this in just about every cartridge . The faster the burn rate the lower the charges will be to get the same velocities up until you blow your gun up with the faster powder trying to get velocities you can only get with a slower powder . Density may play a roll but burn rate is a much larger factor or my understanding of internal ballistics is just flat out wrong .

I wasn't entirely clear. My point is, "Faster" powders do not always equal lower charge weights in loading data or take up more volume. They trend in that direction, but not in a manner that could be used as a rule. Look at Hodgdon reloading data for .45acp and a 230 grain projectile. There are tons of Hodgdon group powders to pick from. Some of the "faster" powders have heavier max charge weights than other "slower" powders (700x vs Winchester 244 and Winchester WST... 700x is a faster powder yet has a higher max charge. Numerous other examples in .45acp data but that's one). Red dot is also a good example, but more in the volume aspect. So they say, 13 grains of red dot fills a 30-06 case more than 50%. It would take more than 30 grains of many other powders to do the same.

My sole point is you can't declare that 22 grains of one powder can be directly compared to 22 grains of a different powder in a pressure equation based off of the burn rate alone. Powders have fillers and other additives, and are not pure nitrocellulose.

[Metal god]

5whiskey : I can agree with that , thanks

[44 AMP]

Why can't you put 40gr of Unique in that 308 case ? 1 it won't fit

I believe it will fit, and I have the "blown up" rifle and scars to prove it!

Can't say for certain if it was actually 40 gr but I know it was over 30.

I was young, and dumber, once...

[Metal god]

Can't say for certain if it was actually 40 gr but I know it was over 30.

You know I don't know either but when I think to how much 5gr of titegroup fills a 9mm case compared to 5gr of Unique , I just figured 40gr would not fit in a 308 case but it might .

[dgang]

So in looking at HiBC's comparison of the .270 to the .30-06 I have conclude that necking down a cartridge to a smaller diameter DOES increase pressure, along with a lot of other factors. Such as necking down a .45/70 to .30 cal. with the same amount of powder may not be a good idea and that if the .223 was NOT necked down to .224 but fired a bullet the same diameter as the case, 22 grs. of H322 would NOT result in high pressures. Thanks to all for input.

[Metal god]

Now I don't mean to be splitting hairs but the question in the OP/title seem to be general to bottle neck cartridges not necking down a case or creating a bottle neck case from a straight walled case and still use the same charge .

Necking down and still using the same powder charge is not the same as just using any bottle neck case as an example , that's apples to … well another fruit . but your right to referencing HiBC's post though .

[dgang]

Metal guard: Probably my analogy between a .357 and a .223 and using different burn rate powders and different weight projectiles was not the best example to use, and a bit confusing. Just wanted to use extremes to demonstrate how a bottleneck seems to increase pressure, similar to putting a nozzle on a water hose. I had read that bottlenecking a cartridge would not effect pressure and wanted to take that theory to the extreme. Thanks for your interest.

[Metal god]

Yep , this thread could have been good with lots of info to be learned . The 458 lot is not a bottle neck cartridge , has a big bore and works at 62'000psi what do we make of that ?

As the bullet moves out of the case neck and into the bore,the volume of the bore behind the bullet is added to the volume of the combustion chamber.


As the powder burns,it produces a volume of gas. Pressure increase is about volume of gas produced vs volume available to occupy.

If you neck down and use the same charge the bullet would need to move down the bore much faster then the original bullet for that cartridge to create that larger volume of space to occupy in the bore behind the bullet . This is where burn rate comes in . To neck down the case to let's say half the diameter then what the original case used . You will likely need a slower burning powder to allow the bullet to get farther down the barrel creating that large bore volume for those gasses to occupy at peak pressure . Fast burning powders peak out faster then slower burning powders .

This is not really about the bottle neck case , it's about the bullet plugging the barrel and it's ability to out run the expanding gases . To use your hose example . It would be like trying to push more volume of water through the nozzle then the nozzle will allow . You force the water fast enough and hard enough and the hose is going to blow regardless if the nozzle is allowing some of the water out because it's not letting enough out at once . You either need to enlarge the hole in the nozzle ( enlarge bore diameter ) or slow the rate the water is entering the hose ( slow the burn rate of the powder ) If you continue to force the water in at a rate the nozzle will not let it out , you're going to get all wet .

Hope that helps
Metal "guard"

[Unclenick]

Dgang,

You seem to be laboring under the false impression that only bottleneck cases are loaded to high pressures. In the old days, if pressure was to be increased by adding powder, they just made the case longer. Today, the 460 S&W Magnum, a straight wall revolver cartridge, has a SAAMI pressure rating of 65,000 psi, the same as the 270 Winchester. People have loaded the nearly straight wall 45-70 to over 50,000 psi in the Ruger single shot rifles for decades. So no, bottleneck don't make pressure higher.

The idea that a small hole in a chamber volume raises pressure in and of itself is like saying the pressure in your air compressor tank jumps when you fill a tire, because the hole you are bleeding the gas through is small. No. That doesn't happen. The pressure just bleeds down gradually. That's all. Use a bigger hose and valve to let the air out and it bleeds down faster, but that's the only difference.

Imagine a case and bullet with a super-fast powder like blank powder in it. You put in just enough to meet 60,000 psi, and because it is so fast that pressure is reached before the bullet moves even a hundredth of an inch. Now you have a pressure vessel filled with pressure and it won't start going down until the bullet starts moving forward. Note that I didn't say what size the bullet was. This could be in a bottleneck case or in a straight wall case. Either way, the pressure is the same.

The above dynamic only changes when we move to slower burning powder. Slower burning powder lets the bullet start to move before the peak pressure is reached. As a result, the pressure is reached in a larger volume, the case volume plus the amount of space added by the bullet moving down the bore. That means we can put enough powder in the case to reach the peak pressure at that bigger volume. It's too much for a bullet that doesn't move or that doesn't move quickly enough, which is why a heavier bullet raises the pressure a charge produces. Once you start counting on that extra space to let you make more gas (which does increase the speed of the bullet over what the super fast powder can give you), you have to be sure it is there. Well a smaller bullet of the same sectional density would move just as fast under the same pressure, but because its diameter is smaller it has to move faster to provide the same extra volume. So you either have to use a bullet with less sectional density or you have to use an even slower powder that gives it time to make enough volume to keep the extra gas you are making from driving your pressure up too high. This is why a smaller bullet in the same parent case (say, a 22-06 verses a 30-06) requires a slower powder and/or a lower SD bullet. Usually it is some combination of the two.

In the case of the straight wall cases, then, reaching a high pressure, if the gun can take it, is just a matter of choosing the right powder burn rate and the right bullet sectional density.

Regarding smokeless powders in general, due to differences in their formulation, they contain anywhere from just under 3600 to just over 5000 joules per gram of potential energy. So yes, sometimes the trend to use slower powder in heavier charge weights than fast powders doesn't play out as expected, and that is generally why.

In this, I haven't gone into the effects of progressive burning verses slow-but-digressive burning powder on pressure and velocity, so there is more for you to dig into.

[dgang]

Unclenick, Metal god: Good points, something to ponder Did like the analogy to air tank and hose. Have to find out more about slow-but-digressive burning rates. I think I understand what you are saying. However, can I assume you are stating it would take a faster burning powder or a longer case with more powder in a straight wall cartridge to achieve the pressure( not in the chamber) and velocity to propel a bullet down a barrel than with a bottleneck? If so, the bottleneck aids in raising velocities. Maybe I am being thick. Have to re-read and think. Thanks

[Don Fischer]

Amazing question! What does any of this have to do with anything?

Ah, think I get it. You want to blow out the 30-06 case to a straight wall case to lower pressure. Go for it. But before you can seat the bullet you'll need to wrap tape around the bullet to close up the MTY space you created around it?

[44 AMP]

Why does a smaller case (.357) with a much faster powder and a heavier bullet result in less pressure? I was told that a bottleneck cartridge does not increase pressure. What goes?

A bottleneck cartridge case does not increase pressure.

you're looking at it backwards. The case design does not increase the pressure. The pressure comes from what we decide to put in the case.

Many, many other factors are at work, and they dictate pressures, case design doesn't. Case design can allow for use of higher pressures, to a degree, but doesn't create the pressure. Only the space to put the powder in.

Does a soda bottle create higher pressure than the soda can? It does not.

[Unclenick]

Dgang,

It is as 44 AMP said. Indeed, my first version of my post started with "you are getting the cart before the horse". You can, in theory, do anything with a long enough straight case that you can do with a bottleneck, but you will have to add enough barrel length to make sure the bullet has the same number of inches of bore travel to pick up speed in. Extending the straight case subtracts from the bore length, so you have to make that up. Bottlenecks are simply a more compact configuration and they keep all the powder closer to the primer, so consistent ignition is easier to achieve.

The thing that determines what powder burn rate you need is the expansion rate. That is, what percent growth the total of the chamber and bore-behind-the-bullet volume grows for each inch of bullet travel. When that is the same, regardless of the shape of the case, the needed powder burn rate and quantity will be the same.

[dgang]

I was told to look into Ideal Gas Rates for a better understanding. This may help to reconcile the idea of volume and velocities. Thanks.

[MarkCO]

The ideal gas law is PV=nRT. P is pressure, V is Volume, n is number of moles (which would be related to grains of powder), R is a constant and T is temperature. It applies only to an ideal gas, or to approximate a real gas. But is does not account for many parameters and characteristics. It is most often used for single molecule gases (which we would not have) at low pressures and high temperatures. But it still can be used to grasp a fundamental understanding of the relationships between the 4 variables.

You would be better off looking at choked flows and supersonic flow theory. There you would gain the understanding those who say that the form factor (bottlenecked case) does not result in a pressure increase can not grasp. In very simple terms, you have water flowing from a hose at full diameter. If you restrict the opening through which the water is flowing...what happens? The pressure IN the hose goes UP and the velocity of the water flowing OUT of the hose goes UP (it is propelled further). If you want the water to have the same velocity without restriction, you must SIGNIFICANTLY increase the volume of water you put through the hose. The "cost" of the restriction is a LOT less than the cost of a higher water volume to get something "far away" wet. Also energy goes as velocity squared, so that higher velocity water does more work than the lower velocity water. You all know this and see this when you wash your car.