# Does a bottleneck cartridge increase pressure?

1. ### dgangMember

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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

2. ### Dallas53Well-Known MemberSupporter

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in all honesty, there are many factors that affect the pressure. powder burn rate, bullet weight, type of bullet, chamber dimensions, atmospheric conditions, temperature, ect., ect., and so on.

trying to wrap your head around all of it, could drive you crazy trying to figure it all out with out having a degree in physics or mathematics.

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3. ### dgangMember

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Thanks Dallas53, just trying to figure it out.

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4. ### Dallas53Well-Known MemberSupporter

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you're welcome. like I said, there are just so many factors that affect pressure in particular cartridge. change one, and the pressure changes.

also, contrary to what some think, smokeless powder does not explode, it burns at a very high rate that creates pressure that accelerates the bullet out of the cartridge and down the barrel. which barrel twist ratios also can have an affect on the pressure rating.

Eg. 223 Rem. with a heavier weight bullet is usually shot through AR type rifles that have twist ratios of about 1:7 to about 1:9. now the same cartridge using a much lighter weight bullet, (like a varmint billet for example.) will generally barrel twist ratios of about 1:10 to about 1:12 or so.

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5. ### OldManMontgomeryActive Member

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The Jury is still out. This question has been debated and argufied for a very long time. Weatherby went to the extreme of designing cases with radiuses on the corners on the shoulder and neck to streamline the inner surface of the case and not interfere with propellent gas flow. (Then they made the barrels with half an inch of freebore - maybe the shoulders were just marketing gimmicks?)

There is no want to compare and find out. There's no such thing as a straight walled .30-06 for example. There is a ".40-06" (same case but necked up to .40 caliber) but the bullet weights are different and the powders used are not the same. So, no real comparison.

Most shooters and reloaders simply don't worry about it much. The loads for bottle necked cartridges don't really give any problems.

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6. ### Dallas53Well-Known MemberSupporter

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very good points Mr. Montgomery.

the Weatherby example being a good one. and another i was thinking about earlier were the Ackley Improved cartridges. case in point, some saw a distinct advantage over the parent cartridge in terms of velocity, and some saw very little.

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Ackley’s improved cartridges were originally developed so you couldn’t take the loads he developed for your new rifle that he had just blueprinted, and stuff them in a less capable rifle and low it up.

Take my favorite the 257 Roberts AI. He was building custom rifles, on completely reworked Mauser 98 Actions. He knew his finished rifles could handle all the pressure he could push at them. The SAAMI standard for the 257 Roberts was 36,000 CPU. He was pushing the hand loads for his customized/sporterized Mauser actions around 52,000, or 55,000-CPU, depending on where each rifle was most accurate, as he worked up the loads for it. He came up with the improved chambers, to assure you couldn’t accidentally load one of his loads into a factory rifle with too weak of an action to handle the pressures, and blow it up. He then discovered that there was less case stretch, and throat erosion with the improved case profile.

A 22-250 AI, 25-06AI, or 243AI, won’t give any velocity improvement over the parent case, because the parent cartridges are already, relatively high pressure rounds, that are over capacity for the bore. The AI chamber will give you some decrease in throat erosion though.

The other direction, the 35 Whelen AI. It will have little effect on velocity, because increasing the shoulder angle, and blowing out the case a bit, gives you a very small percentage increase in chamber volume. It may result in better accuracy, and possibly longer case life, just because it results in a better shoulder to head space off of.

alsaqr and Dallas53 like this.

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You might review the various gas laws in a chemistry or physics book

If all things are equal
Pressure 1 x volume 1= pressure 2 x volume 2.

Below is the Ideal Gas Law.
It was first stated by Émile Clapeyron in 1834 as a combination of the empirical Boyle's law, Charles's law, Avogadro's law, and Gay-Lussac's law.[1] The ideal gas law is often written as PV = nRT
Where
P = pressure
V = volume
n = number of moles of gas
R = Universal gas constant
T= temperature in degrees kelvin

Yes this can be strange if you have not studied it before. It is typically covered in the introductory classes and only requires algebra to cover the introductory material. If you go into any real depth of study a background in calculus is needed.

Anyway you can get a little intro just by reading the wiki article on the ideal gas law.

Last edited: Dec 17, 2018
9. ### Dallas53Well-Known MemberSupporter

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or better yet, just follow recommended recipes from published current load data books. and let the ballistics technicians work out all the physics and math. seems much simpler to me.

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All the physics and math? This is theory you cannot learn much theory in a data book.
You simply need a little understanding of high school algebra and be able to read. Then you read about 10 pages in a book. Most people are not harmed by a little basic education.
Like a boss once told his employee when the employee had to complete a difficult task. The employee complained that finding the answer was difficult.
The boss said "If it was easy we would not need you". Sometimes solving a problem and learning a new skill is a reward all of its own.

Last edited: Dec 17, 2018
11. ### dgangMember

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Thanks Iraeload2, Ideal Gas Law. This the kind of thing I was looking for.

12. ### Dallas53Well-Known MemberSupporter

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First of all, I have used some form of algebra in some capacity for many years in my career and hobbies. second, part of my job description was finding answers and solutions to difficult problems. third, comparing the pressure readings of a straight-walled pistol cartridge to a bottle-necked rifle cartridge is even beyond comparing apples to oranges.

And if a person were starting from scratch and designing, or engineering a new cartridge, or coming up with a new wildcat cartridge, then all of this might have some useful meaning. But for the average reloader, not so much. if a person can follow written instructions, adhere to some simple procedures, and work in safe manner, they can reload a cartridge. no physics or theory is required.

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The bottle neck cartridge can not increase the pressure. Guns that use bottle necked cartridges typically have longer barrels, and more time to develop pressure.

The burn rate of the powder is based on the case volume, the bore area, the sectional density of the bullet, and the length of the barrel.

Basically, as the case volume increases, relative to the other factors, you move to slower powders.

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14. ### OldManMontgomeryActive Member

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You'll pardon me, Jigs, but I see several problems with this post.
First is bottle neck cartridges are not routinely longer than straight sided cases. I have a .458 Winchester Magnum with as long a barrel as any other common rifle I have.

The second statement you make IS very close to the reality. The term to which you refer is 'Expansion Ratio' and is (simply put) the interior of the case volume compared to the volume of the barrel. A .40 caliber barrel of twenty-four inches has more internal volume than a .22 caliber of twenty-four inches. If the cases are the same volume, the larger barrel will have a bigger expansion ratio. A larger expansion ratio requires faster powder.

A larger case volume does not always require a slower powder. With the same caliber barrel, a larger case will make a small expansion ratio - which mandates a slower powder. A .458 WM uses surprisingly fast powders.

To complicate matters, higher chamber pressure typically demands a slower burning powder than lower pressures.

I'mm not sure if it is still in print, but a book titled Firearms Pressure Factors (Lloyd Brownell, PhD) has it in great detail.

15. ### ChainfireWell-Known MemberSupporter

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Think of it like a water hose. If you leave the end unrestricted it produces high volume but low pressure. The same hose restricted at the end produces high pressure and low volume. In either case the same energy is expended.

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16. ### JTJWell-Known MemberSupporter

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I was going to say Ask a plumber.

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17. ### JTJWell-Known MemberSupporter

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Straight wall cartridges require much different burn rates than bottle necked cartridges. You are comparing apples and oranges. Pistol powders are regularly used in bottle necked cartridges for REDUCED loads and cast bullets. You can imitate 22lr with a 223. I used to load .308 100 grain cast bullets in a 308 rifle using pistol powder with a kapok filler. Very quiet. Good round for rabbit at 25 yards or so but then so is a 22lr.

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Figuring it out is not burden to everyone. If you feel that way why don't you let the original poster speak for himself and you can quit responding to posts you feel are too complex to study.

19. ### dgangMember

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Inquiring minds "just gots to know". Firearms Pressure Factors sounds like something I need to read. Started this thread when loading .357 and .223 ammo and started to wonder why the big difference in pressure when the .357 case was smaller with a heavier bullet and a faster powder. Just wanted a few more facts. Part of the enjoyment of reloading, nothing more. Good shootin' to all of ya'.

20. ### JTJWell-Known MemberSupporter

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Interesting thing with the 357. It gains a lot of velocity in a longer barrel as opposed to the 9mm. Efficiency plays a big role. The 124 gr 9mm gains about 150 fps in a 16" barrel while the 357 125 gr gains 400+ fps. 2000 fps for the 125 gr. Now consider instead of a 357 you load a 30 caliber in a sabot. Single shot rifle time. That 125 30 will maintain velocity a lot better than the 125 357.

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