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 Post subject: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 8:05 am 
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I've always understood that a smaller case with the same components in it will have a higher breech pressure but don't understand exactly why. I am thinking of this scenario with the difference in volume being taken up by a slightly smaller wad of the same type in the smaller case and it's a generic question not aimed at any particular recipe.

Any thoughts/ideas?


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 8:08 am 
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Actually, you answered your own question: smaller volume. The generated gases don't have as much space to expand.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 8:49 am 
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Shoot2reload wrote:
Actually, you answered your own question: smaller volume. The generated gases don't have as much space to expand.


In the scenario I mentioned the difference in shell size was accounted for in the different wad size so the space to expand would have been the same pretty much.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 9:02 am 
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You have changed two varibles, the case size and the load (a different wad), so there is no way to accurately guess what the result might be.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 9:16 am 
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As Shoot2reload said, it's all about the space inside the case. It's the same cause that makes the pressure inside a 410 more than that inside a 12ga. If your net combination of components results in less space, your pressures will go up.

That said, I can't quite see why it's relevant to shotshell reloading, where all the space inside the hull is filled with powder, wad and shot. I know it's significant and can be critical with rifle ammo, where the slight difference in case volume between a commercial 308 case and a military 7.62 NATO case can have perceptible effects (tho' of course you're talking about vastly higher pressures there).

I'd have thought the important factor in shotshells would be the stiffness/springiness of the wad; does it squash up easily, thereby presenting less resistance to the expansion of the gases? hence, presumably, the issue of wad pressure with fibre wads, where too much wad pressure can increase chamber pressure, because (I assume) the wad's already been compressed and therefore can't readiuly compress further.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 9:41 am 
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I believe it is related to the shape of the powder - take a CF AA hull and then a straight walled federal type hull. In one case the powder is sitting in the hull in a 'truncated egg shape' and in the other more of a disk. I don't know which will have higher or lower pressure - but the flame front in the powder and the resulting combustion will be different and should result in different pressure traces.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 12:16 pm 
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Because!

This is a good of answer as you're likely to get.

Do you know anything about building high performance combustion engines? Milling heads, high rise pistons, compression ratios. All done to slightly decrease the capacity of the cylinder, but at the same time increasing the compression ratio and ultimately the pressure of the combustion gas driving the piston. This ultimately results in more power/torque. Kind of like a shotgun shell.

I don't know that the shape of the base wad has any particular effect on the situation, but, I don't know that it doesn't either. Does it provide for a shape charge effect?

I dunno.

I think "Because" is still the best answer. We know that it makes a difference, it's been measured time after time. Why, isn't so important here, the fact that it does is.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 12:59 pm 
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Last edited by dogchaser37 on Tue Feb 07, 2012 4:04 pm, edited 1 time in total.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 3:45 pm 
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lemming wrote:
As Shoot2reload said, it's all about the space inside the case. It's the same cause that makes the pressure inside a 410 more than that inside a 12ga. If your net combination of components results in less space, your pressures will go up.

That said, I can't quite see why it's relevant to shotshell reloading, where all the space inside the hull is filled with powder, wad and shot. I know it's significant and can be critical with rifle ammo, where the slight difference in case volume between a commercial 308 case and a military 7.62 NATO case can have perceptible effects (tho' of course you're talking about vastly higher pressures there).

I'd have thought the important factor in shotshells would be the stiffness/springiness of the wad; does it squash up easily, thereby presenting less resistance to the expansion of the gases? hence, presumably, the issue of wad pressure with fibre wads, where too much wad pressure can increase chamber pressure, because (I assume) the wad's already been compressed and therefore can't readiuly compress further.


Fibre wads generally generate less pressure because the seal around them is less efficent than a plastic powder cup.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 9:42 pm 
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Lets take a look at what the OP has asked and lets assume the same primer, powder, amount of powder the same wad, same payload and crimp depth, even the same plastic for the hulls and hull length.

The difference is one hull has an internal capacity of 1 cubic inch the other hull .9 cubic inch.

Remember that breech or chamber pressure is at it's peak before the wad leaves the hull. The primer is fired off it ignites the powder pushes wad and shot ahead some 'x' distance along with collapsing the wad.

Since we are using the same powder and amount of powder and primer the whole combination is going to produce the same amount of propellant gas. Since the .9 cubic inch capacity hull has a smaller internal capacity, the gas does not have the same room as the one cubic inch capacitiy hull, which raises the pressure, but it also makes for a more vigorous burn, which in turn raises the temperature, which raises the pressure even higher in the smaller capacity hull.

This is where rrd should step in a say a few words.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Tue Feb 07, 2012 10:42 pm 
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I missed this part.


It has been my experience that a hull must have a smaller I.D. to call it smaller capacity, just cutting off a little bit of hull and calling it less capacity isn't the same and will not change pressures much, if at all.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 11:35 am 
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Dogchaser, I think you are right when you say cutting off a little of the hull probably won't have much effect on pressure. Cutting off a significant amount can cause a drastic rise in pressure. I don't know what your educational background is & this is not directed at you, or anyone else for that matter but internal ballistics is a very complicated subject that requires a through knowledge of physics & the associated sciences to understand. I am no expert on internal ballistics & I can not tell you why some loads are seemingly unaffected by changes in volume & some are. The best I can do is an educated guess. From basic hydraulics, & this is just scratching the surface of the variables involved, the force exerted against a piston (or wad) is proportinate to the area of the wad. (Cross section). The more area the gas has to work against, the less pressure it takes to get a load moving & keep it moving. Also, the initial resistance to forward motion is a biggie. It always takes more energy to start an object moving than to keep it moving which I will guess has a lot to do with why max pressure occurs in the chamber. Sure, it is great to understand why things happen but most of us (me included) have to be satisfied in knowing what happens from experiment & relying on what the experts tell us. I have been told by ballistic experts that volume can increase pressure dramatically. It does not always do so as I am sure you know. I don't quote people without their permission but if you want to discuss the issue with him, I will ask him if I can give you his number. Also, I am not saying this guy has the last word on the subject but he is quite knowlegable. The fact that there will always be disagreement amoung experts is almost as sure as taxes. For those interrested in the accademics of the subject, I would suggest studing the work of Ed Lowery but that will require a through knowledge of differential equations at a minimum.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 11:41 am 
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There are a lot of factors that determine chamber pressure but the OP asked about case capacity. I tried to keep my answer focused on his question.

I am confident that I have given the correct answer and the major reason why. Now are there more factors involved? Yeah but now you start to run down the muddy road, which isn't necessary for this discussion

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 12:06 pm 
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OK dogchaser, only problem is you can't isolate case capacity as the single variable. To quote a fellow I corresponded with:,"a shotgun load is a system". There is no linear relationship between case volume & pressure nor is there one formula that ties it all together.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 12:54 pm 
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geometric,

Quote:
There is no linear relationship between case volume & pressure nor is there one formula that ties it all together.


That isn't what I wrote or commented on. I think you are misreading what the OP asked. I completely isolated case capacity and have spoke only on why a smaller capacity case produces higher pressures.

Please only comment on what I wrote not what you wanted me to write. I understand about the other variables, please don't include them in this conversation, as they aren't important in this topic.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 1:53 pm 
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For those of us that are not science majors (I have a Finance Degree). Does the powder burn act something like water through a pipe. Where all things being equal, a smaller pipe with the same volume of water pushed through would have a higher pressure?


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 3:20 pm 
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Itaintover,

Not quite, but you're on the right track!

It, the water per se, wouldn't have a higher pressure, but it would require a higher pressure to push the same volume through the smaller pipe. You could push the Mississippi river down a 6" pipe if you could push hard enough.

Interestingly enough most powders tend to burn faster under pressure so the higher the pressure, the faster the burn, the faster the burn, the higher the pressure, the higher the pressure, the faster the burn, the faster the burn, the higher the pressure, the higher the pressure, you get the idea.

Water and pipes do not have that property, it's an exact science. Powders are different, some are "worse" than others in that situation, the unknown is what gets you into trouble with the experimentation.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 5:02 pm 
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Interior ballistics gets a little complicated, but for the first order approximation if you cram the same amount of gas into a smaller volume you will generate higher pressure.


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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Wed Feb 08, 2012 10:29 pm 
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Thanks rrd, glad you were listening.

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 Post subject: Re: Why does case size affect breech pressures?
PostPosted: Fri Feb 10, 2012 11:10 am 
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The question was why does case size affect pressure. Simple question, very complex answer. Last night I was reading an article in an old reloading manual. Most of the data is for obsolete components but the information was undeniably ballistic fact. The only simple answer is because it is bigger, ie: has more room. For those seeking a better explanation, all I can suggest is they do some reading. Although they have certain things in common, internal ballistics is not hydraulics. The author of the article I read did not attempt to answer the question of why but instead concentrated on how.
It mostly has to do with the burn rate of powder under certain conditions. Too much volume & it is hard to get powder to burn efficiently. Too little volume & the components can't move fast enough & pressure goes up rapidly. Obviously, the case volume should idealy be such that the burn rate of the powder is optimum. Tappered target cases are more efficient with light target loads. Large, straightwall cases generally yield better ballistics with heavy hunting loads at lower pressure. Hope this helps but as I stated before, I am not a ballistics expert, therefore, I rely on what those that are tell me. The physics here is beyond my knowlege & also the scope of a reloading forum.


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