Cast bullet accuracy and trailing edge failure

[303Guy]

This also gets me to wondering about the folks who have reported shooting unlubed bullets with a cream of wheat filler. If this theory is true, I suspect that you might be able to do the same thing with a slow burning charge of powder.

I have found that boolit bases are peened and cupped with high loadings of slower powders just like DrB has predicted!

I'll need to check but I seem to think that with higher load densities and only a small amount of wheat bran, the base peening and cupping is less than without. It is hard to assess due to the damage sustained by the boolit on impact in soft sand. I'd have to use damp rags as a catch medium to see for sure.

Rebate base showing cupping.


Strong cupping with pronounced edge rim.


All my photo's seem to support DrB's hypothesis.

[DrB]

Thanks for sharing, 303guy. Always great to get a look at reality!!!

Btw -- you would think the peening would show a variation in impact marks (if individually discernible) from the centerline radially out. I would expect more normal/rounded impacts in the centerline, and more oblique impacts and/or streaking or nothing as you progressed outward toward the bore.

In a couple of your pictures it almost looks like there might be a distinct central circular area that's been peened more/more normally? Can you discern any radial variation in the peening?

Lastly, I'm curious about the paper patching not separating on the peened loads... was this consistent or perhaps a shot or two fluke? If consistent, do you think the peening had an effect on the failure of the patch to peel (I don't see why it should -- seems like the patch should still separate fine except maybe at the trailing edge)?

Best regards,
DrB

[303Guy]

The peening appeared consistant. All the loads with wheat bran filler seem to retain at least the rear section of the patch at lower pressure levels and the base ring at higher pressure.



These came from boolits like this one.

[Doc Highwall]

I am not sure that it is peening from the powder as much as I would believe that the powder was compressed against the base of the bullet upon ignition.

The powder when first ignited starts burning at the back of the case near the primer, pushing a wad of powder against the base of the bullet leaving a imprint on the base of the lead bullet, before all of it is ignited within an inch or less of travel down the bore.

To say that the powder peened the base of the bullet it would have to be traveling faster then the bullet like a sand blaster.

The powder and gasses cannot travel faster then the bullet while it is in the bore to peen the base of the bullet, but when the bullet leaves the muzzle the powder gasses can travel faster but only for a short distance.

[felix]

Primers fire at greater than the speed of sound. ... felix

[DrB]

I am not sure that it is peening from the powder as much as I would believe that the powder was compressed against the base of the bullet upon ignition.

The powder when first ignited starts burning at the back of the case near the primer, pushing a wad of powder against the base of the bullet leaving a imprint on the base of the lead bullet, before all of it is ignited within an inch or less of travel down the bore.

To say that the powder peened the base of the bullet it would have to be traveling faster then the bullet like a sand blaster.

The powder and gasses cannot travel faster then the bullet while it is in the bore to peen the base of the bullet, but when the bullet leaves the muzzle the powder gasses can travel faster but only for a short distance.

I'm not sure that the velocity of the solids would be great enough to leave an imprint from impact, either, vs just leaving an imprint from acceleration.

However, I do think the solids can travel faster than the bullet up until they reach the bullet base and turn outwards towards the bore. I think they'll turn outwards because those particles impacting the bore surface will loose velocity and travel rearward relative to the bullet base.

[303Guy]

Either explanation could do it I should think. Considering that My boolits have a fibrous wad under them it would seem feasable that the centre of the wad is being forced up against the boolit base with more pressure in the centre being transmitted to the boolit. Powder might behave in the same way while still burning at launch.

The wad formed by wheat bran whould have drag on the sides which might explain the higher force transmission at the centre.

There has got to be a degree of gas impingement onto the boolit base at a molecular level, in the bore as that is how gas pressure is transmitted but whether the burning powder granules are carried with a greater velocity than the boolit itself would be speculation on my part.

I had actually thought the base cupping was caused by the boolit being swaged down with gas pressure acting on the base causing the sides to drag rearward. The sample without filler seems to indicate otherwise.

So, I'm open to suggestion here.

Oh, I have samples of gas checked boolits that also show base cupping. Worse still is that the base edges were uneven after firing!

Here's a boolit with edge 'cupping'!



It's the patch tail impression.

[felix]

Next time you let water out of the tub, notice the circling motion. Same with boolits. The cavity appears AFTER the boolit starts moving. ... felix

[303Guy]

Accepted but does it form in the thoat as the base is being sized or further down the bore when the boolit has had time to 'flow' as lead does? Or perhaps a little of both? And is it caused by peaning or simply unbalanced pressure?

[felix]

Unbalanced pressure, really. Think tornado, typhoon, etc. ... felix

[DrB]

RE gas impingement, there is nothing that necessarily requires there to be the same gas pressure at the base of the bullet and back in the bore gas behind a slug of solid particles (or vortex of solid particles). But reading again, I don't think that is what you were saying. The solid particles will, along with gas particles, transmit driving force to the bullet base (obviously they are transferring some momentum to it based on the impressions left from the force of their impact!)..

RE vortex, I'm missing something here? The pressure in the center of a vortex is reduced, not higher... so if a vortex spinning rifling wise about the axis of the bore centerline were having a significant impact, it would cause the bullet base to experience more pressure at the edges than the center (reverse cupping!). Am I misunderstanding the vortex axis you are proposing, felix?

Also, rifling doesn't constitute much in the way of vanes to impart angular momentum to the flow.... angular momentum is conserved just like energy, mass, etc, so angular momentum has to come from somewhere.

Seems to me there really has to be a ring vortex in the bore behind the bullet because of the boundary conditions, particularly when you get farther down the bore and bullet velocity has picked up. With a high fraction of solid particles in the flow, intuitively I think it could cause a significant lateral gradient in the pressure on the bullet base. The particles may compress and slug up or they may fluidize... I'm not sure if they slug up though it would have the same effect RE base cupping?

Man, more great pictures from 303Guy. That's really neat to see the paper patch shape swaged to the bullet base. What're the concentric rings on the one on the left? Is that unfired vs. fired, and the bullet is from a reamed cavity mould (the concentric rings are tool marks in the nose of the cavity)? Also, I wonder if you'd get any cupping at all in that particular example if you trimmed the tail end of the paper patch? It looks like you can see the impression left from creases in the PP on the boolit base... seems to me the circumferential crease in the paper at the base corner could also result in extrusion of lead into the crease from the paper being pressed into the lead? Conversely, I wonder with a paper patch like that that goes over the base if you couldn't get cupping irrespective of much solids in the flow due to the same effect (the creasing of the PP at the circumferential edge)?

[303Guy]

Its a nose pour mold and the rings are the base plug that I didn't machine very smoothly. It's an unfired and fired example.

Both of these were the same bar the filler.



I'm wondering how much effect the wad has in forming the cupping. The wad would tend to centre the pressure due to the drag on the edges. Both tese were low pressure loads.

I have had base distortion with a flat based boolit and gotten rid of it using a rebated boolit but I put that down to swaging in the throat of the two-groove with 80% bore to groove area.



I'll try with setting the patch just a fraction forward of the base and see what happens.

[357 Voodoo]

RE vortex, I'm missing something here? The pressure in the center of a vortex is reduced, not higher... so if a vortex spinning rifling wise about the axis of the bore centerline were having a significant impact, it would cause the bullet base to experience more pressure at the edges than the center (reverse cupping!).

DrB

Bernoulli's principle May have some effect in in this conundrum and if this were the case than with the lower pressure in the center the velocity of the gas and any particulate matter would be increased.

[303Guy]

That's the lateral pressure. This a closed system with an expanding combustion chamber and burning matter all the way down the length of the bore.

[DrB]

357 voodoo... Yeah, I'm just not seeing the bore coaxial vortex as a big thing, though? Pressure in the center should be lower with such a vortex. Also, I don't see anything that would create a strong coaxial vortex in the first place.

I can see a ring vortex with a mixed phase flow having a significant effect on bullet base pressure distribution, but I'm a bit out of my comfort zone with mixed phase flows as I never specifically studied them.

[DrB]

Here's an example derivation of velocity distribution in a simple vortex.

http://www.desktop.aero/appliedaero/...l/vortexv.html

You can see there is a (mathematical, not physical ) singularity approached as r goes to zero, and that pressure is decreasing with radius.... (this is why sometimes you can see vortex cores in humid conditions. The air cools enough from adiabatic expansion that water vapor condenses out).

A ring vortex is what you see in a mushroom cloud. Gases rush in at the bottom to the middle, turn upwards, curl over at the top, and rotate thusly. The center of rotation takes the form of a closed ring. In the mushroom cloud analogy the cloud head vortex part would be pushing up against the base of the bullet. The backwards rushing bore walls (in a bullet-fixed frame of reference) would tend to strip particles breachwards away from the bullet base, promoting the vortex circulation and decreasing pressure at the periphery of the bore as compared to the centerline.

[303Guy]

Have a look at Jeff's photo. It shows dragging at the rifling impression. This type of dragging could conceivably contribute to base cupping.

[DrB]

Is it dragging or swaging though? If you engraved the same bullet by hand (maybe would have to distribute the force over the whole base) would it show the same marks?

I think it might show those tabs independent of any gas pressure at all... and that would indicate it wasn't drag pulling the bullet base back but the base edge getting squished out by the rifling?

[303Guy]

I think it might show those tabs independent of any gas pressure at all... and that would indicate it wasn't drag pulling the bullet base back but the base edge getting squished out by the rifling?

Agreed. I used the term 'dragging' because that's where the displaced material went and I would expect there was dragging friction involed too. I thought it might shed some light on the processes involved. I suspect there several factors at play and that which factor (or combination of factors) dominate may vary.

The fact that a fibrous filler ripped Jeff's case mouths off indicate to me that there is a great deal of lateral friction involved.

[DrB]

I didn't see Jeff's original post... hm. Different materials can matter a lot RE friction, but I see your point. Does the lube groove tell us anything? Can't tell if it's narrower just because of the riflilng cutting into the narrower depth of the groove or if there is reverse "dragging" of material forward.

I'm going to have to start recovering some bullets, 303Guy. This is fascinating, but looking at pictures (even these really nice closeups) isn't the same as holding 'em. Part of that's conceptual, I think, but a lot of it is perceptual... I forget what the numbers are, but a greybeard tool & die maker once told me the relative tolerances a person could see vs. feel... you can feel the difference between a very very small dimension, much smaller than you can see with a Mk 1 eyeball.

What're you using for your test tube?

I was thinking of trying to recover some of the NOE225107 37 grainers I've been shooting at 3200 fps using a 4'x8' by 4" thick piece of sandwiched polystyrene foam, backed by a gel bucket. At 3200 fps I'm expecting anything recovered would be too mangled unless I slowed it down with 8' of foam first... so hence the foam. I'm thinking sandwiching it with plywood will probably keep it ricocheted back into the middle of the foam if it starts to fish-hook out (although such a short bullet with such a wide meplat should be fairly unlikely to do that, at least until it's already going really slow). Seems to me the foam approach should work for recovering the bullet with less damage.

Best regards,
DrB