Do bottleneck cartridges increase pressure?

[44 AMP]

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.

I think I grasp it pretty well, I just think it doesn't apply to cartridge cases the same way it does to a hose. Yes, I understand about fluid dynamics, gas laws, venturis, and other things. The hose nozzle analogy is accurate, for a hose, but not so much for cartridge cases, because there is no changing the "nozzle size". Also, unlike a water hose, where you are using one standard system pressure and varying the nozzle size, cartridges have different pressures, created by the type and amount of powder we put in them. Each one is different.

Why does a .357 Magnum run at 35K and a .223 at 55k (or 62k?) not because one is a straight case and the other is bottlenecked, but because of OTHER FACTORS that dictate what the safe and allowable pressures can be.


The guns they are being used in is the major factor.

[F. Guffey]

Do bottleneck cartridges increase pressure?

I am the fan of the pump start, I want my bullets to have that 'jump start', there is something that is unsettling about the bullet setting at the lands when the pressure starts to climb.

It is one of those for certain things; I know the pressure is going to increase; what I do not know is if the bullet is going to get out of the way before the pressure gets to the point it renders my rifle scarp.

I like the ideal of the running start, I want my bullet past the rifling before the bullets knows the riflings are there.

F. Guffey

[F. Guffey]

And then there is that thing with 30/06 and 338/06, I wondered and then started on a 280/338 and someone asked 'WHY? And then I mused over the question and the logic behind adding .051" to the length of the case over the 338/06.

And I wondered if it had anything to do with .7854.

F. Guffey

[dgang]

Thanks MarkCo: I will be looking at choked flows and supersonic flow theory and other equations I have found on the net. Might keep me busy for a bit. LOL

[MarkCO]

No problem dgang. I love the stuff, but most find it to be other than I do.

[MarkCO]

Originally Posted by 44 AMP

I think I grasp it pretty well, I just think it doesn't apply to cartridge cases the same way it does to a hose.

Sorry, had to laugh. I, like, most engineers I work with have a saying that follows when someone says "I think". It goes something like "Liar, Ignorant or too kind". Don't take it personal. Sometimes the math proves the assumption wrong.

Sorry the analogy did not help you, but be assured, the mathematical models I have run accounting for all relevant variables, both as a consultant for, and in doing my own product design on gasses from gunpowder and solid rocket propellants have clearly shown that form factor does impact pressures, and constriction of flow most definitely increases pressure. If I had made a mistake on a few of them, it would have made national news.

[dgang]

F. Guffey: You mentioned necking up a .30/06 to a .338/06. If you were to do so with the same weight projectile to the same velocity, would it take more, less or the same amount of the same powder? Seems to go to the crux of the matter.

[Unclenick]

Originally Posted by MarkCO

You would be better off looking at choked flows and supersonic flow theory. There you would gain an understanding of those who say that the form factor (bottlenecked case) does not result in a pressure increase cannot 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?

If the case was open to the air and the powder evolved gas fast enough or if you had a practically unlimited supply of gas, as your garden hose has with water, you would develop more pressure in the more confining smaller-necked case of two chamberings sharing the same parent cartridge (say, 30-06 and 25-06). The problem is the gun is not open to the air when the bullet is in it propelled. The bullet winds up being the gas flow rate limiter, especially in the first couple of inches of bullet travel when the pressure peaks. We know this because the muzzle blast spheres in shadowgraphs get out ahead of the bullet, initially, indicating the gas pressure allowed for greater gas speed than the bullet allowed it to have. Also, if the bullet were not the dominant factor in limiting that flow rate, having a longer barrel would not increase velocity as there would be no significant pressure behind it to accelerate it further.

You may find a small pressure difference due to case mouth narrowing by the time the bullet nears the muzzle, but the difference is just a few percent. I think QuickLOAD's model does a pretty good job of predicting this. It gives you both muzzle pressure and the pressure in the chamber at the time the bullet base gets to the muzzle. For a 100 grain bullet in the 25-06 fired with Varget to a peak pressure of 60,000 psi, with a 24" barrel the pressure at the muzzle is 21.5% lower than at the chamber. For a 30-06 with a 144-grain bullet (to match sectional density) the difference is 17.7%. So you get about 3.8% lower muzzle pressure due to the narrower bore. At the peak pressure, when the bullet is still moving slowly, the difference is going to be much less. On average you might see a 2% difference in bullet acceleration due to it. Not much.

[5whiskey]

You mentioned necking up a .30/06 to a .338/06. If you were to do so with the same weight projectile to the same velocity, would it take more, less or the same amount of the same powder? Seems to go to the crux of the matter

I'm not guffey but i can answer the question. It would take more powder in .338-06. The same way it takes more in 30-06 than in 270.

[dgang]

You may find a small pressure difference due to case mouth narrowing by the time the bullet nears the muzzle, but the difference is just a few percent. I think QuickLOAD's model does a pretty good job of predicting this. It gives you both muzzle pressure and the pressure in the chamber at the time the bullet base gets to the muzzle. For a 100 grain bullet in the 25-06 fired with Varget to a peak pressure of 60,000 psi, with a 24" barrel the pressure at the muzzle is 21.5% lower than at the chamber. For a 30-06 with a 144-grain bullet (to match sectional density) the difference is 17.7%. So you get about 3.8% lower muzzle pressure due to the narrower bore. At the peak pressure, when the bullet is still moving slowly, the difference is going to be much less. On average you might see a 2% difference in bullet acceleration due to it. Not much.

Unclenick: Just out of curiosity, in your comparison of a 25-06 and a 30-06 load what would the optimum barrel length be?

[Unclenick]

5whiskey,

Note that in a larger diameter projectile a same-weight bullet has lower sectional density than its narrower counterpart. That means it takes less pressure to achieve any specific rate of acceleration. Thus, to achieve a specific velocity, less powder is needed with the wider, lower SD bullet.


Dgang,

I don't know what you mean by optimal. If you read the Houston Warehouse experiments you will conclude that 21¾" empirically seems to be best for rifle accuracy regardless of chambering, but clearly not best for long range shooting. If you mean, what barrel length will extract the maximum amount of energy from the powder? that will be the length at which acceleration just reaches zero. For a .22 LR, that is 16 to 19 inches, depending whose load you fire, the high speed being at the longer end and standard velocity being at the shorter end. After that, the bullet gradually starts to slow down in the bore due to friction, though not enough to matter in a 24" .22 LR barrel. For high power rifle cartridges with their much greater powder capacity, though, a barrel long enough to achieve that is impractically long for many cartridges. With long barrel friction activated, QuickLOAD puts it at 45.6 inches for the 30-06 load, above, dropping to 45.0 inches with a 175-grain bullet and the same powder loaded to the same pressure. Switching to IMR4350, a slower powder loaded to the same 60,000 psi, it grows the barrel to 47.8", so you can see this doesn't change a lot. Maintaining accuracy with a barrel that long whipping around would be difficult, at best. It only gains about 100 fps over a 32" Palma barrel, which is long enough for most folks.

[dgang]

Thanks Unclenick, I was referring to max velocity. My guess was around 4 ft. for a .30/06, so I was pretty close. Had just read an article about velocity from shotguns being fastest at 18" to 20", that would put my BT99 at a much slower rate than I had thought. Have never been able to get a good reading from a Chronograph, probably because of shot column, wad, gases, etc. Thanks for your answer.