General cartridge design

3202 Views 17 Replies 6 Participants Last post by Rich Z, Jul 8, 2004

Rich Z

Discussion Starter · Dec 7, 2003

A while back I talked to some guys about pressure signs and the topic of the bulged case came up. Seems that most of the time, you will see a bulged case right near the head on a case. Now the brass isn't any thinner there than elsewhere, so we got into disussing what was going on to cause that.

Well the consensus was that at the instant the primer is ignited and the first bit of powder ignites, not only is the weight of the bullet keeping the pressure confined, but also the weight of the powder that has yet started burning is a factor. As added evidence, barrel makers will tell you that throat erosion on a barrel is often caused by unburned powder sand blasting that front part of the barrel and starting point of the rifling. So this seems to indicate that a good bit of powder is being thrown up through the barrel before it finally gets lit afire.

Take a .50BMG cartridge, for instance. When the primer first ignites, there is not only the 650 to 800 grain projectile sitting on top of that first pressure wave, but there is also around 220 grains of powder sitting on top of it as well.

So is the basic design of a centerfire cartidge just wrong, or at least highly inefficient? Wouldn't it be better to have a design that would do a controlled burn of the powder from the FRONT (near the projectile) which would tend to utilize only expanding gas to push the bullet out the barrel? Since the powder would be burning back towards the head of the case, there would not be any case of the weight of the unburned powder being a factor in the pressure curve.

Yeah, this is all ivory tower thinking, and trying to come up with a new cartridge design that would work this way would probably be very difficult to do. New technology has advanced to electronic igniting systems now, and the impact initiated primer really is no longer a necessity.

How were the caseless projectiles ignited? Weren't they just basically a block of powder with a primer embedded in them? Whatever happened to them anyway?

Anyway, just some idle speculation on a Sunday afternoon....

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1 - 18 of 18 Posts

.45 COLT

· #2 · Dec 8, 2003

That "front ignition" idea has been done. Rocky Gibbs, who developed the Gibbs wildcats, played with it, as have a few others. A significant increase in muzzle velocity is seen. I have no information on what it does to pressure, but I reckon it would affect it.

A tube threaded into the primer flashhole carries the flame to the front of the powder charge, or at least near the front. Difficult loading and questionable results.

I wouldn't tell Remington about the advantages of electronic primers. They lost big on that so far.

DC

Aslan

· #3 · Jun 22, 2004

not to mention, that unless the cartridge is full of powder, then the powder is on the bottom of the cartridge, not against the primer.

What you actually have is the powder on top and towards the back burning first, the turbulence form the burning gases should cause the rest to become somewhat suspended as they ignite.

You'd almost be better off with a solid charge of propellant with a shaped hole through the middle of it.

Powder would burn evenly on all sides, following the hole (which probably would be more star shaped than round)

of course, the propellant thus describes a solid fuel rocket.

Just wondering out loud...

:devil:

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41mag

· #4 · Jun 22, 2004

Wasn't the supposition that powder will ignite at first the back & then then front-meeting somewheres in the middle-the reason assumed for kabooms in some vintage rifles & large bore revolvers years ago.I don't remember all of the article but I think that the jist of it was that A(ignition) + B(ignition) = C(big ignition)ie.a little more than is safe.
I seem to remember Skeeter Skelton writing an article about loading lg cases with fast burning but low volume loads.IIRC he suggested using a filler(cornmeal?)to keep the powder volume compressed against the bullets base.

To be actually on topic,why couldn't a round be designed to have a)no primer pocket at all.solid base,& b)put the priming compound behind the bullet base but above the powder charge.A thin wafer maybe. & c)use electricity to ignite the primer.Maybe have a couple of electrodes imbedded at opposing points in the chamber?

Aslan

· #5 · Jun 22, 2004

Onions are good, especially vadalia sweets.

:devil:

andy

· #6 · Jun 26, 2004

what doesn't the modern cartridge do,

that NEEDS doing and CAN be done with a firearm,hmm? NOTHING, that's what. Spend more time gunhandling, shooting and working on hand to hand fighting skills.

Rich Z

Discussion Starter · #7 · Jun 26, 2004

Well, for one thing, if the initial pressure spike wasn't having to move the rest of the powder along with the projectile, I suspect that higher velocities might be achievable with less pressure. Surely it would reduce the initial pressure spike. I suspect there is a reason that most over-pressure signs are noted at the head of the case. If you can bypass that "hump" by more efficient cartridge design, it could open up a new level of performance.

Take the case of the .50BMG cartridge. Upon initial ignition of the powder by the primer, not only is the gas pressure wave trying to move a 650 grain (approximately) projectile, but it is having to move whatever remains of that initial 225 or so grains of powder in the case between the primer and the projectile.

Not only that, but from what I understand, a large part of the erosion caused at the beginning of the barrel at the throat is by the sand blasting effect of the unburned powder acting upon it at every shot.

And thirdly, if the majority of the powder is burned BEFORE it gets into the barrel, rather than while it is actually IN the barrel, it may substantially increase life of the barrel be reducing the direct contact of the interior of the barrel with the flame of the burning powder.

All supposition on my part, of course, but it's fun to think about such things.

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KJUN

· #8 · Jun 26, 2004

41mag said:
I seem to remember Skeeter Skelton writing an article about loading lg cases with fast burning but low volume loads.IIRC he suggested using a filler(cornmeal?)to keep the powder volume compressed against the bullets base.

...but them some other people have TRIED for a long time to MAKE this happen, and couldn't. It convinced them that it is just a myth.

Personally, I don't believe it is very likely at all, but it might be possible, so I am very careful when it comes to low charges in big cases. I think this is one of those VERY rare cases (that might in fact be due to something unrelated) that has been blown up into some type of urban myth.

So, nobody hase been able to repeat this reliabvly in controlled conditions, but it MIGHT be a possible RARE event.
KJ

KJUN

· #9 · Jun 26, 2004

Re: what doesn't the modern cartridge do,

223 fan said:
that NEEDS doing and CAN be done with a firearm,hmm? NOTHING, that's what. Spend more time gunhandling, shooting and working on hand to hand fighting skills.

I bet they said the same thing in the 1700's, too!

KJUN

· #10 · Jun 26, 2004

Rich Z said:
Well, for one thing, if the initial pressure spike wasn't having to move the rest of the powder along with the projectile, I suspect that higher velocities might be achievable with less pressure. Surely it would reduce the initial pressure spike. I suspect there is a reason that most over-pressure signs are noted at the head of the case. If you can bypass that "hump" by more efficient cartridge design, it could open up a new level of performance.

I never really thought about it, but I always assumed (and I know what they say about assuming....lol) that it had something to do with the chamber design. The brass case swells to fill up the chamber, so I figured that the chamber wasn't supportin the case as well in that region to be able to accomodate ALL factory loads or something. It may be WAY off base, so I figure I would ask and see if anyone knows if that might be obviously incorrect or possible.

It just seems to me that the chamber would have to be "looser" at the beginning for proper feeding and chambering, and a "loose" area would be the least supported. This would mean more possible brass deformation.

KJ

andy

· #11 · Jun 26, 2004

if they were as stupid as u r, maybe, but

MOST saw the advantages of rapidfire, silence, lack of corrosion, etc. Ben Franklin called for the militia to use xbows, and Napolean had REAL problems with Prussians armed with air rifles, dummy.

KJUN

· #12 · Jul 1, 2004

I'm not sure about all of this. For example, a slower burning powder would have MORE powder and, well, burn slower. In my mind, this implies that MORE powder would be pushed against the bullet (if this is true) than a faster burning powder. So, a slower burning powder would be worse than a faster burning powder? This sounds backwards from what I would believe.

Does anyone have any evidence of the effect of powder burning speed on this one way or another?

I just don't think that mass of powder initially in between the bullet and the expanding casses from the combusted powder is that much of a real issue. It just doesn't seem to make sense to me.

I could be VERY wrong, though.
KJ

Aslan

· #13 · Jul 6, 2004

I beleive the burn rate of the powder has more effect on the impulse than anything else.

You could have two powder charges that impart the same velocity on the bullet, but one does so in a shorter period of time. the one taking longer is, of course, the slower burning of the powders which should have a lower overall pressure spike.

the faster burning powder should have a bigger spike and would be the one, to me, that would probably cause the most wear on the gun.

but, I could be wrong...

:devil:

41mag

· #14 · Jul 6, 2004

IIRC the article I was thinking of was actually regarding straight wall rifle cartridges.Now that I really think about it he might have even been talking about smokeless loads in cases designed for black powder.That would explain the seemingly excessive case volume for a modern powder & why some type of filler would be preferred.
I apologize for typing out an unfinished thought.It has been @ least 10 yrs since I've read any S.Skelton article.Guess my memory is fading....

:bomb: :beer:

Rich Z

Discussion Starter · #15 · Jul 7, 2004

Wish I had the time to talk more in this thread about this.

But I think volume is important. Fast burning powders reach peak pressure faster because more powder gets burned within the volume of space it has available. With slower burning powders, the first grains of powder converted to gas are pushing the remaining powder forward and soon enough, the projectile as well. This gives the gas pressure more volume to expand in, which reduces the pressure. In a completely sealed volume, both fast and slow powders of the same volume would reach the same peak pressure and hold it until some external force released it. But the faster powder would reach the peak sooner than the slower powder. With fast powders, if the volume increase caused by the remaining powder and projectile moving forward doesn't cause a quick enough reduction in pressure, then bulging case heads and flattened primers result. In a worst case situation, you get a kaboom event.

Heck, check a reloading manual for the difference in powder loads using the same powder but for different weight projectiles. If you can reduce the weight the initial flash of powder has to start moving significantly, then perhaps MORE powder can be used without creating excessive pressures. Which may cause increased velocities without the pressure penalty.

But I'm not a physicist or cartridge designer, so my thoughts may just be all wet. From what I remember back in my youth with playing with homemade fire crackers and such, the rule of thumb was to leave one third of the volume empty to get the most bang for your buck. And of course, the fuse came in through the TOP of the cracker to ignite the powder from the top. So maybe setting off firearm cartridges that way would be worse, rather than better, then the current way it is done.

Wouldn't be the first time reality slapped me in the face with cold hard facts!

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KJUN

· #16 · Jul 7, 2004

Rich Z said:
> In a completely sealed volume, both fast and slow powders of the same volume would reach the same peak pressure and hold it until some external force released it.

That could only be true if the density of the two powders were EXACTLY equal.

Aslan

· #17 · Jul 8, 2004

plus, the volume would change as the bullet began to move. but the pressure curve would be sharper for the faster burning powder...

if nothing else, this is one of those discussions that are interesting, and actually civil!

Rich Z

Discussion Starter · #18 · Jul 8, 2004

I believe the purpose of different burn rate powders is to try to match the maximum safe pressure with the expanding chamber created by the moving projectile. In a perfectly designed match of gun barrel, powder burn rate and projectile, the maximum safe pressure would be reached as quickly as possible, and maintained right up to the point where the projectile exits the barrel. This is difficult to do because the volume the pressure is contained in is expanding as the amount of powder transformed into gas is increasing. So it is a balancing act.

From what I understand, this is why loadings for non moly coated bullets don't normally give the same velocity when used with moly coated bullets. The increased lubricity of the projectile actually allows the projectile to move more easily down the barrel, which means that expansion chamber for the pressure gets larger faster then a non moly coated projectile will allow. So the pressure curve is different. Generally pressure is LESS because the expansion chamber gets bigger, more quickly than before.

The original thought behind this thread was that the fact that the ignition of the powder starts at the back of the powder charge increases the complexity of this formula in that initially the expanding gasses have to move the remaining powder as well as the projectile. So the maximum mass that needs to be moved is at the initial ignition of the powder charge. The mass gets reduced as more of the powder burns, but it is that initial pressure spike that needs to be accomodated for in any load you may develop.

Plus, from what I understand, significant erosion of the throat of the barrel takes place from those unburned particles of powder producing a sand blasting effect until they get transformed into gas.

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