Do lead boolits get soft when obturated?

[leftiye]

The title says it all. The thought crossed my mind today that when you break down the crystalline structure of antimony /lead alloys, they lose their hardness. So, what happens to the base of a boolit when you hit it so hard with a pressure wave that you make mush out of the crystalline structure? Sounds like soft lead to me????

[Bass Ackward]

The title says it all. The thought crossed my mind today that when you break down the crystalline structure of antimony /lead alloys, they lose their hardness. So, what happens to the base of a boolit when you hit it so hard with a pressure wave that you make mush out of the crystalline structure? Sounds like soft lead to me????


Can absolutely. That's why we match our hardness to the pressure, both the level and the rate at which it comes up. That's what we are controlling. Same with a GC.

That's supposedly the basis for the hardness formulas. But change case capacity, barrel length, bore diameter, powder speed, bullet weight, lube quality, etc, etc, and you will need to create a new formula for that gun caliber combination either up or down.

[leftiye]

So what does this do to obturation theories? I guess other factors keep leading down after the mechanical squeezing destroys the crystalline structure maybe? If we keep pressures below the yield strength of the alloy, this all becomes moot, but there are gazillions of loads that totally ignore the yield strength, many of which are accurate.

[felix]

Time is involved. The boolit is out of the gun long before you can measure a change. That's my bet. How long? Don't know. Need to shoot a few into the sand pit and measure immediately at the boolit base as well as where it expanded into the sand. Why not the entire length of the boolit while you are at it. ... felix

[Bass Ackward]

So what does this do to obturation theories? I guess other factors keep leading down after the mechanical squeezing destroys the crystalline structure maybe? If we keep pressures below the yield strength of the alloy, this all becomes moot, but there are gazillions of loads that totally ignore the yield strength, many of which are accurate.


Leftie,

Obturation can have nothing to do with expansion from softening (what's the scientific term?) of the base. Obturation can be from slumping as the base tries to pass the nose. So it depends on how the seal occurs. There are variables that can aid or prevent that. Ever hear of Elmer's square vs round groove debate? Elmer wanted his base to perform the seal as opposed to the bullet losing the balance he designed and molded into it by slumping.

There are gazillions of accurate loads for softer lead. I push the soft envelope all the time. But .... you .... will encounter a limit in everything. Do I find 15 loads from which to pick the most accurate one? Not on the edge. I am lucky to get one.

But I thought your question was hard lead that WENT soft. There is a BIG difference between a load that you plan for, and take steps to make shoot with soft lead as compared to a harder bullet load that you exceed the pressure rating for the mix that goes soft under pressure. Understand the difference?

Finding that balance between all those variables is where the experimentation comes in with your firearm. But to your question.

I had two 44s that were water dropped. One was 14 BHN from 50/50 WW / pure. I wacked it with a 5lb sledge and the new (displaced) nose measured 9 BHN.

I had one WDWW at 24 BHN on the nose. I wacked it and the new nose measured 24 BHN.

Both were measured within a minute, both rearranged the molecules or whatever.

Now that ain't chamber pressure, but it might invoke some thought.

[Harry O]

I believe there are two parts to "obturation". Every metal (including lead) has an elastic range and a plastic range in the stress/strain diagram.

The elastic range comes first. As long as you do not exceed the top of the elastic range (the yield point), the bullet will return to its same size and shape (except for the rifling grooves, which are obviously beyond the yield point) when it leaves the barrel (when the pressure is removed from the base of the bullet). That means to me that there is NO change to the hardness.

When you exceed the elastic range, you get into the plastic range. When that happens, you have exceeded the yield point and the metal will take on a permanent deformation. It will be larger in diameter and shorter than it originally was. And it will never return to its original size and shape, even when it leaves the barrel.

In that case, there will be permanent changes, particularly to the base of a plain-base bullet, which harms accuracy. It stands to reason that it would be work softened, too.

I found out when recovering and measuring 40:1 lead 41 Long Colt bullets that they expanded to the full diameter of the grooves (the groove marks were evenly marked from the rear of the bullet to where the nose curves away from the barrel). However, the diameter of the recovered bullets were about halfway between the original diameter and the groove diameter. That meant they shrunk after leaving the barrel -- but NOT all the way back to the original diameter. The springback was from the part that was still in the elastic range. The part that did not springback was in the plastic range.

[leftiye]

Bass,
I don't distinguish between mechanical, and pressure, slumping or whatever cauing the softening. I think that if whatever re arranges the metal, it destroys the crystalline structure.

Harry this would agree that softening definitely happens during plastic deformation, there is yet a chance that it would happen too during the elastic situation as re arrangement of the lead even temporarily might destroy the crystalline structure.

As for time, there was a thread here wherein someone quoted a college prof as stating that the work softening progressed at about the speed of sound. That would travel the length of most boolits while they were still in tha barrel. The few thousandths of lead near the surface would take about one twelve millionth of a second.

[Bass Ackward]

Bass,
I don't distinguish between mechanical, and pressure, slumping or whatever cauing the softening. I think that if whatever re arranges the metal, it destroys the crystalline structure.


Harry,

Nice simple explanation for a guy like me. That's why I can't get to wrapped up in the science. It just confuses.

See if a guy was going to push more pressure and he wanted a better seal, then the logic would be to go softer as you pushed harder to ensure this plastic state the entire length of the bullet. When in truth, we are trying to avoid that plastic state by hardening to avoid it.

That's the problem. We all endeavor to overcome that which we create. And if we aren't .... flexible enough to do what each stage requires, we can't make it work. A rifleman doesn't see things the same with cast as a handgunnner looks at it. Most handgunners are looking to seal and a rifleman generally already there. And then a rifleman that shoots 40-1 is different from a benchrest guy shooting 22 BHN and up.


Leftie,

The only thing that has a crystalline structure as I would call it is something that is heat treated. HT generally softens back over time. A lead mixture that derives it's hardness from composition always has that composition and hardness. Lino that has been pushed to the plastic state will be 22 BHN till the cows come home.

This is why we endeavor to harden. But this is just my theory why some guys prefer to blend as opposed to heat treat. You see this today in benchrest comp data put out by the CBA. Some guys HT and others do it chemically.

But this for me is the downside to cast. Theories. People will tell you that you need lino or better for 2000 fps plus. Or for accuracy. I use lino, but @ 1000 fps and below. And I push the envelope because I am just to dumb to understand that I am not supposed to do it.

Off to the woods!

[trk]

Good question. Perhaps achademic, perhaps practical.

Obturate means to seal by expanding.

So the hardeess (hence strength of the alloy as well) is directly linked to the process. As said before, the hardness is something we can (should) control and relates to obturation.

SO, what if it gets soft or not in the process? I'm sure we could measure the hardness when it's still hot from it's moment of fame streaking down the bore.

Is it more or less accurate and does it lead more or less are the significant quesitons.

[leftiye]

B.A. All lead/tin/antimony/arsenic/phosphorus/copper alloys (in whatever composition) develop crystals (crystalline structure), and it is the crystals that develop any hardness beyond the hardness of the original dominant metal. Heat treating causes optimal growth of the crystals, and freezes them, not allowing them to break down due to slow cooling.

Trk, You are right about the definition of obturate, maybe. I've always thought it was the expansion of the bullet either caused by firing pressure or impact with something. Regardless, the question relates to whether boolits work soften when obturated or slumped or otherwise deformed. There is a possible conflict between hardening the boolit and then work softening when fired if pressures are too high. I have read in quite a few places that once the crstals are broken down that lead alloy has the hardness of pure lead if these crystals are broken down mechanically.

[trk]

My first introduction to the word was the 'obturation disk' on the breech block of the 155mm self-propelled howitzer - it expanded from the presure of the charge being fired and the stainless disk squeezed the neoprene disk outwards to seal the breechblock.

In the same respect the base of the bullet (or boolet) is pressured by the expanding burnign charge and it deforms and presses outward into the open area of the throat and then is squeezed down to engage the rifling. Somewhere in the process either or both the front bands or the base band seals the hole wherein the bullet is going.

Yesterday at a match I observed an interesting feature on t3o different targets that I'd shot at 100 yards.

The first was with a 45-70 with 28.5 grains of 4198 and a tuft of dacron - the bullet was breech seated - stuck entirely into the rifling save for the last band.

The second was with the .458WM same charge different bullet (harder, about the same weight and different mould) - loaded into the case.

The .45-70's had no black ring around the holes in the target.
The .458WM's had a distinct blac ring around the holes in their target.

Both bores ended up being about the same level of clean.

That leads me to believe that the .458WM bullets did not seal (obtruate) as quickly as did the .45-70's - blow-by causing the 'soot' that the bullet carried to the target.

[jhalcott]

I know it has been done BUT I can't remember the results. Take a Linotype bullet and smash it with a hammer. Then take the hardness of it,comparing that to the original BHN. Do this with a heat treated bullet and find out IF the "mechanical obturation" had ANY effect at all. Maybe putting them in a 200 degree oven before the hammer treatment MIGHT work to soften them.
On the other hand WHY should I care? IF(big IF) the bullet does what I want it to does it matter if it gets fatter(or shorter) in the process?.

[trk]

Replace the hammer with a sizer. (Sizing isn't much different than the bullet (oversize) squeezing down to fit the bore.)

Check hardness before and after.

The bullet going softer or not is academic - but knowing that is the basis for further understanding, therefore important.

[leftiye]

Trk is right. This thread to me is all about the understanding thing. I don't have an axe to grind here, only an intrigueing question. To my mind, the boolit should be softened whenever the metal is worked in any manner (if a boolit obturates, then if all else is done correctly it should seal too, but it should soften too. Perhaps I should have used the term expansion).

As previously stated, this opens up a question as to if hardening, or using hard boolits is not compromised by this work softening. Obviously, soft boolits can and are made to shoot, and just as obviously hard boolits do make a difference. And after all since all can be made to work, I guess we're all welcome to use the method we want to. BUT, this phenomenon (couldn't think of any other way to express it- don't know if it comes up to the term) probably does occur, and that may be something we want to take into account.