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View Full Version : Crimps, I feel like making more noise.



44man
05-20-2005, 04:46 PM
All of us hear that a case has to expand to let the bullet out and keep pressure down. Very common with BR rifles and tight chambers. (but is it fact?) But I have questions. I have been trying to imagine myself in the case at the moment of ignition. Does the entire case expand all the way to the mouth right now, leaving the bullet hanging free while the bullet is still in the case, or is it progressive, expanding the brass as the bullet moves out of the way? Kind of makes a guy wonder, doesn't it?
This brings up the hard crimp on revolver cases. Does the pressure expand it open before the boolit leaves? I hardly think so. I say the boolit has to force it open and if the lead is too soft the crimp will scrape/size the lead making the boolit smaller or leaving grooves that the gas can get into and cut the boolit. Making some tests today with cases that had both jacketed bullets and cast boolits shot from them, I found I could get a cast boolit back into the case that had the condom bullet shot from it, but could not get one in the case that had a cast boolit in it. There was still a lot of crimp left on these cases. This can only mean my boolit was damaged when shot! Does a harder piece of brass cause more damage and open groups?
Take a loaded cast boolit and remove it with an inertia bullet puller so you can see the damage and the amount of crimp left on the case. The crimp is almost exactly the same amount as is left on a fired case. Is this a cause for poor accuracy and barrel leading?
Maybe this is why I find hard boolits shoot better! Also a good reason for gas checks.
Does anyone consider this damage when they say to use softer boolits for "bump up". Is "bump up" enough to eliminate the damage from the crimp? I don't know! I want to make all of you guys think!
I was shooting water dropped WW metal for this test.
Starmetal, I await your thoughts, and I do respect them!

StarMetal
05-20-2005, 05:09 PM
44man

First off on those two case mouths you compared saying the one that held the jacketed bullet would accept a cast bullet, but the one that held the cast would not. Are you positive that both had the exact same pressure level? (ballpark, doesn't have to be down to NASA measurements). Maybe the jacketed one had a much higher pressure and it really pushed the entire case against the chamber walls.

Next about the cast bullet pushing the crimp out of the way. If you put a stick in the ground and you just push it with your hand, it leans as far as you push it. If you kind of whap it with your hand hard it will lean further or either fall down. I think a cast bullet starting on it's way does so very very rapidly, so I think pretty much of the crimp is laid out like hitting that stick hard and fast with your hand.

On case expansion, who's to know exactly what is going on there. If the gas expanded the case immediately, wouldn't it also just blow by the bullet being everything is happening so fast? Or...is moving the bullet forward an easier task for the gas pressure then expanding the case?

Think those over.

Joe

beagle
05-20-2005, 05:36 PM
In my opinion, and this is just my opinion, the amount of crimp aids in building pressure before the crimp breaks loose and allows the bullet to move.

The bullet moves and at the same time, a small amount of gas escapes around the bullet. We know this happens from looking at the pictures in Phil Sharpes book of the Thompson being fired with a Cutts comp. Of course, this is a FMJ 230 grainer with a milspec crimp and the asphalt like sealant.

I beleive that crimp does have a great deal of affect on initial velocity and certainly on consistent ignition.

No less authority that the Army at one time used aluminum shear wires with it's 2.75" rockets. I know as I installed enough of them. This shear wire retarded the rocket in the tube until a much higher pressure built up enough to shear the wire and allow the rocket to move. Initial speed was enhanced and most of all, this caused improved accuracy because the high speed working in conjunction with the beveled fins cause it to spin faster.

Now, what does this have to do with cast bullets? A lot I'm thinking. We have loads like the .38 Special wadcutter and breech loaded .32-40s that are loaded with a propellent that more or less pushes the bullet down the barrel with very little pressure, relatively. You can see this if you look at some of your weaker .38 Special loads after firing. The small pressure isn't even enough to iron out the crimp completely at times. Then, we have loads such as the .44 Magnum with H110 and WW296 that require a heavy crimp. This retards the bullet and allows pressure to build much as in my rocket analogy. The burn is more consistent and we get better SDs, higher and more consistent velocities and hence better accuracy.

We also get some blowby during the first nano seconds before the bullet moves forward and seals the bore.

Just my take on the situation and no flames intended toward anyone./beagle

Bass Ackward
05-20-2005, 06:27 PM
You can see this if you look at some of your weaker .38 Special loads after firing. The small pressure isn't even enough to iron out the crimp completely at times. Then, we have loads such as the .44 Magnum with H110 and WW296 that require a heavy crimp. This retards the bullet and allows pressure to build much as in my rocket analogy. The burn is more consistent and we get better SDs, higher and more consistent velocities and hence better accuracy.

We also get some blowby during the first nano seconds before the bullet moves forward and seals the bore.

All,

Now if you guys want more food for thought, let's look at shooting bullets slightly larger than the throat and what it does for us.

1. Case tension is eliminated because the bullet replaces neck tension for ignition. Except that the bullet hardness is easier to make and keep consistent. So you can shoot 50 year old brass, annealed or not, mix and match, and it doesn't matter.

2. You don't have to worry about crimp or crimp damage to the bullet because the bullet goes no where until the brass expands and let's go of it all together. Crimp importance is minimized and in fact loads can be worked up that use no crimp at all just relying on the bullet for good ignition. You might have to play with diameter or hardness a little, but it can be done.

3. You get very little gas blow by because seal is almost instantaneous compared to other methods that require some form of deformation. ES can come down to rediculously low numbers using virtually any powder within a common sense range, loaded to any pressure / velocity level you want.

4. Any time you can eliminate variables required for successful shooting, you increase the odds for accuracy.

5. Because you don't have to over crimp and size your cases down one whole caliber to increase neck tension, you actually can extend brass life to rediculous levels. And you need less powder to generate the same pressures. So you shoot cheaper.

6. This method minimizes the effect of bullet design and bullet weight allowing as good of ignition and alignment with light bullets as if you were shooting 350 grainers. Yet pressures can be less of an increase than if changing to a magnum primer from a standard. Consistency is the name of the game.

StarMetal
05-20-2005, 08:51 PM
You have to be one good dang shooter to detect minute differences in a pistol or revolver when you're not using a Ransom Rest. Big differences yes, minute no.

Joe

Iron River Red
05-20-2005, 09:25 PM
Bass,

Would it not be logical to expect a loss of pressure while the bullet is in the forcing cone of a revolver and this same gas that escapes there, could be creating a sort of reverse shock wave that flows backward into the case while the bullet is engaging the rifling in a closed chamber?

All this is theoretical of course, but we know that the powder burns faster as it developes pressure. There fore the pressure a bullet sees my rise and fall slightly several times before the bullet actually leaves the case.

To this add the "ripple" this must induce into the case and you can see the bullet may be gripped and released several times before being expelled.

There may also be enough ripple to allow the bullet to escape the grip on the first cycle.

All this happening in a fraction of a second...

David R
05-20-2005, 09:39 PM
Acouple of my cents. I could never get as much of a crimp on a jacketed boolit as a cast, the groove is much deeper.

The back of the boolit has LOTS of surface area for the charge to push on compared to what can get by the boolit and force open the crimp.

I am only guessing that the peak pressure is after the boolit leaves the case, so It could then be pushing out the crimp.

Just some thoughts, good reading too.

Iron River Red
05-20-2005, 09:47 PM
It seems to me the gas expanding would exert it energy initially against the large area of the be and then "roll" off until it overcame either the tension of the case wall or the bullet started to move. Either way, the case will see a linear increase of temperature and pressure.

Now, will this temperature "rise" be sufficient in degree or have enough time to reduce the surface tension of the brass.

I don't remember which will expand faster. lead or brass? What then?

44man
05-20-2005, 10:49 PM
Lots of good thoughts here. Let me answer a few questions. First the jacketed bullets had a lighter load then my cast boolits. I also use a minimum crimp on my cast boolits so they are the same as I get on jacketed. I did not put a deep crimp groove on the boolit when I made the mold.
The point of an oversize boolit sounds interesting. In fact I hand seat my BPCR loads and do not crimp them. I depend on the boolit being held in by the rifling leade. My boolits are also oversize. I never thought to apply this to the revolver, have to think on it because it will take a two diameter boolit, part to enter the throat so normal seating depth and chambering is achieved, then the larger rear portion to contact the start of the throat to hold it in. Need a chamber cast to get the proper dimensions. Good idea, I will work on it!
As far as the .38 special not ironing out the crimp, those are dead soft boolits. I am sure they are sized down by the crimp but being so soft, they must bump up when entering the forcing cone. Kind of like a chunk of putty. A hard hunting boolit would have trouble bumping up that much.
A thought about the crimp springing back after the boolit leaves, can't spring back that much! If ironed flat, it will stay flat, just springing back a few thousandths.
We have a good start here, keep thinking and I will answer what I have tried so we don't get a lot of repeats.

Bass Ackward
05-21-2005, 07:26 AM
Bass,

Would it not be logical to expect a loss of pressure while the bullet is in the forcing cone of a revolver and this same gas that escapes there, could be creating a sort of reverse shock wave that flows backward into the case while the bullet is engaging the rifling in a closed chamber?

All this is theoretical of course,

IRR,

How are ya going to proove it? That is what Taylor throating was supposed to help. But I find Taylor throating only effective in transitioning a larger diameter bullet to a smaller bore. Less than .002 difference between throat and groove diameter with good alignment, and Taylor throating is virtually worthless and in fact can cause leading problems, with moderately hard bullets.

Look. The only tool I have is a chronograph. Based upon my limited knowledge, I can expect more pressure (what ever the cause) will show up as increased velocity with a given powder. People argue all the time about secondary pressure spikes and still no one can accurately describe the process. So I play the odds of history that go back a few years now and trust that I will be OK when I pull that trigger. So far it's workin.

But I am amazed about how people object to the thought of larger diameter cast bullets in a wheeler but swear by them in a rifle. Seems to me that it is rather nieve to expect obturation in a rifle throat but not in a handgun chamber. So I ask what is the difference between an undersized bullet and a larger diameter one? Answer. None. Both go through deformation until they fill all the free space in the chamber and both are exactly the same diameter when they leave the throat.

25,000 psi is 25,000 psi. But a rifle case has more case capacity to start a bullet off slower and doesn't usually peak at 25,000 until the bullet is over 500 fps and 2" down the bore. And here people will tell you that bullet will slump and you need to fill the throat. But a wheeler hits 25,000 psi quicker because of smaller case capacities and powder speed. The bullet starts off much faster to peak pressure. And here they think only the base expands to seal and only after it has moved into the throat.

So what do I believe is the difference? The larger bullet seals first and goes through less over all deformation because it is almost there from the begining. It keeps the lube on the bullet instead of being blown forward. Which is why Elmer made such a deep, square groove was to hide lube from .... passing gas. And that is also why Keith type and shooting of undersized style bullets tend to lead more than other methods. And why hard undersized bullets lead more because they never obturate (deform) the entire length to seal at lower pressures.

So this gives my answer to 44mans question about does a bullet start to move before it leaves the case? Yes. It moves outward and crushes the brass against the chamber before the bullet even moves. And you are sizing down your cases and crimping to help produce this deformation process so you can get shot to shot uniformity.

carpetman
05-21-2005, 07:40 AM
Bass Ackwards--You ask what is the difference between an undersized bullet and a larger diameter one? Usually it's just a few thousandths difference. Glad I could help.

45 2.1
05-21-2005, 07:48 AM
On the subject of remaining crimp. The case mouth will expand to the point of letting the bullet go and spring back some, in a crimped state it is smaller in diameter than the case body. The case body also expands and springs back some! It will be larger in a fired state than the case mouth that was crimped. This is why you have the condition of remaining crimp.

Oldfeller
05-21-2005, 08:10 AM
Take a firmly crimped bullet and case with no powder -- just primer. Tape a little ball bearing centered over the primer. Point the slug at a bundle of rags and wack the primer with a hammer. POP !!!! Out comes the slug ... worst case with no expansion of the case taking place at all, just the slug bulling its way out of the tight crimp.

Go look at the slug and see if it gets torn up. Not very much at all, huh? Haven't you ever pulled some slugs with an inertial puller and looked at them? Once again, not much real damage to the slug when measured as a diameter. The tiny bit of band rounding and scraping is cosmetic, not functional.

When it first goes off, it is the primer driving the entire show for a pico-second or so. IMHO, the bullet doesn't even budge before at least some of the powder starts to burn, so you have constantly increasing pressure taking place both overcoming bullet inertia and when it goes over the expansion PSI expansion of the case walls takes place.

I'd vote for the case walls being tight to the bullet at primer ignition and before the bullet finished totally leaving the case the case walls have started to move outwards some. Can't prove it of course, that's why it is just opinion.

It was fun though, for the first cup of coffee on a Saturday morning at home.

Oldfeller

45 2.1
05-21-2005, 08:59 AM
More fun, what happens in what order: Case wall expands, pressure overcomes bullet inertia and bullet starts moving pushing crimp open, pressure expands bullet, expanded case walls let propellant gas down sides of hard bullet letting gas cutting occur.

My take on this is that in pistol brass that the case wall expands some before the bullet moves allowing an under size bullet to misalign and start a little crooked. In thicker, stiffer case necks the bullet might move first. I think it would be easier for the pressure to move annealed brass than it would move a bullet many times heavier.

BD
05-21-2005, 09:28 AM
I think the crimp groove design, and the size of the drive band imediately behind it, has a lot to due with whether the bullet is damaged by squeezing out of the crimp. I much prefer the Keith style of crimp groove which is square on the front and then tapers out to full diameter to the rear. This design also allows you to crimp as you seat without shaving any lead using a taper crimp.
As too boolits over chamber diameter in a revolver, How'ya gonna get them loaded in the pistol? My bullets are fit to my chambers, and they reach to just shy of the cylinder face. If they were any fatter I'd have to drive them into the cylinder. BD

beagle
05-21-2005, 10:04 AM
Damage to bullets as far as diameter reduction is sometimes caused by neck tension on seating. I've seated some .30 Carbine cast .312" bullets and had an "Ah ****!" and had to pull some. These were measured and ran a bit over .308". So, this means that these bullets were sized down by the neck tension on the case.

On pressures in the case during ignition. There will be two and maybe even 3 pressure spikes if we measure chamber pressure on an oscilloscope.

The first will be the primer firing. This will push a portion of the powder against the bullet base.

The second will be as the powder ignites and builds pressure and the bullet starts to move.

The third will be as the bullet takes the rifling./beagle

felix
05-21-2005, 10:45 AM
Beagle, I had no idea the compression of a boolit could be THAT bad! ... felix

beagle
05-21-2005, 11:33 AM
Felix....A soft alloy bullet like I use ina pistol case and a expander designed for .308" jacketed bullets leaves the .30 Carbine mouth pretty small. The cases, especially the militay brass is very thick. When you seat a soft bullet in this combination, it swages it right down. I was surprised too.

The other affect of this is that you'll actually "bump" the nose of the bullet as well due to the effort required to seat. With a press like a Rockchucker which I use, with the amount of leverage, you'll never feel it either.

That's one reason to constantly measure stuff before and after if you want to know what's really happening with your loaded cast bullets./beagle

Bass Ackward
05-21-2005, 03:56 PM
As too boolits over chamber diameter in a revolver, How'ya gonna get them loaded in the pistol? My bullets are fit to my chambers, and they reach to just shy of the cylinder face. If they were any fatter I'd have to drive them into the cylinder. BD

BD,

Part of the problem that we try to deal with is terminology. In a rifle chamber, the bullet rests in the throat. In a wheeler, the bullet resides in the chamber if you keep your ammo close to factory specs. Often you will be able to chamber a bullet several thousanths over your throat diameter. But there is no way to chamber a bullet larger than your chamber diameter in a wheeler. I choose to shoot .001 to .002 over my throat diameter which still allows me to seal rapidly enough to get great ignition and still chamber rounds when my chambers get dirty. My drive band and olgive are cut to just kiss and follow the taper angle on the throat. But you have to have a cast to measure and get this right. Too hard to do with a slug.

This is why Elmer said that Lyman ruined his design when they cut down his front band. Elmer had specially made S&Ws for him and these guns allowed him to actually seat the front band into his throat until they shortened it. Then his bullet had to move forward and up the front taper ramp into his throat and of coarse Elmer realized this was deforming his bullet.


44man,

<<I never thought to apply this to the revolver, have to think on it because it will take a two diameter boolit, part to enter the throat so normal seating depth and chambering is achieved, then the larger rear portion to contact the start of the throat to hold it in. Need a chamber cast to get the proper dimensions. Good idea, I will work on it!>>

This is the advantage to Taylor throating. It provides "a" transition diameter down to groove diameter. A two diameter bullet also provides centering guidance as the bullet enters the cone and I find it helpful if you don't have Taylor throating OR the difference between throat diameter and groove diameter of the barrel exceeds .002 for harder bullets. With less than 14 BHN, I can go as far as .004 over before no distinct advantage is gained. But for handguns with reasonable throat to groove relationships and good alignment (>.002) Taylor throating seldom produces better results. Same with a two diameter bullet. This is also why some revolvers benifit from fire lapping and some don't. It all depends on the method of choice for shooting and or the dimentional relationships of the gun.

44man
05-21-2005, 08:13 PM
This has proven to be a great post with all the good thinking and answers. It has given me a whole new bunch of thoughts to keep me awake at night. A lot more experimenting to do too.
I agree with boolits getting sized a lot with tight case tension and is the primary reason I use hard boolits in my hunting loads to limit this. A soft boolit is in trouble with tight brass.