Author Topic: Question about fragmentation  (Read 430 times)

Alan

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Question about fragmentation
« on: January 31, 2019, 05:25:24 AM »
On H.A.M., you posted a ballistic gel photo of your .25 caliber hollow point bullets, fragmenting on impact. It interests me that they do this. What they should be doing, is mushrooming. To me at least, this tells me they're a bit too brittle.

I will admit, I'm not an expert on lead bullet casting, albeit I used to do a lot of it. Nonetheless, I've had this "effect" happen to me, when there was a high differential between pour vs. mould temperatures. A friend at the time, suggested I look at the pours with a high-power eye loupe. Sure enough, I could see finite lines in the casted bullets. The "cure" was to keep the mould laying across the top of the lead pot when not actually pouring. I think it was Kevin who recently suggested doing this.

Comments?


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Alan

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rsterne

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Re: Question about fragmentation
« Reply #1 on: January 31, 2019, 08:17:13 AM »
Fragmenting of very soft bullets (those were 1% tin) can be controlled by the depth of the Hollowpoint pin.... Those moulds were done by Erik, and he made the HPs large and deep.... I have found in the testing I have done that if the HP cavity goes all the way back to the rear of the nose, where the bullet is full diameter, that fragmenting is common.... Since they are Varmint bullets, I don't care....

A larger diameter HP makes the bullet expand at a lower velocity, or expand to a larger diameter at a given velocity.... A shallow cavity limits the extent of the mushrooming, and since the lead can only stretch so much without breaking, if the cavity is deep enough, the nose tears, and fragmenting is promoted....

I don't believe that really soft lead-tin alloys change hardness depending on quenching (or pouring into a cold mold)…. Those containing Antimony certainly can and do....

Bob
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Bullfrog

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Re: Question about fragmentation
« Reply #2 on: March 18, 2019, 05:40:37 PM »
In my long clay video, I demonstrated a little bit how with my tire weight alloy, I get violent fragmentation when the impact velocities are around 850fps or higher. When my impact velocities are lower, I get a traditional mushroom. I used to think my alloy was brittle, but now I think its just very soft.

I'm fine with the fragmentation. I still get about the same amount of penetration into my clay at 50 yards, and I like knowing that at lower velocity (further distances) I'm still getting a good mushroom.

Alan

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Re: Question about fragmentation
« Reply #3 on: March 19, 2019, 05:05:52 AM »
You might be correct.

We need to realize, that pellet lead alloy, like glass, is a super-cooled liquid. Both these alloys act like Silly Putty. Pull it slow, and it stretches. Pull it fast, and it tears apart. If you want to demonstrate this, put a small wad of Silly Putty on an anvil, and hit it with a hammer.

Perhaps the term—elastic limit—applies?
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Alan

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rsterne

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Re: Question about fragmentation
« Reply #4 on: March 19, 2019, 03:35:45 PM »
I think it is much simpler than that.... Lead has a very low tensile strength, and an even lower force required to make it yield (bend, ie expand).... When the combination of velocity and hollow point size is large enough, the front of the HP opens, and as long as the bullet keeps going, and there is any HP cavity left, it keeps opening, stretching the lead around the front of the opening.... Once it gets big enough, and the front has stretched enough, the tensile strength is exceeded and the lead begins to tear.... You get petals of lead forming.... The deeper the HP cavity is, the longer the petals get, and as they fold back over the base they can tear off, resulting in fragmentation....

Basically as you add other metals to the lead to increase its hardness, it also increases the tensile and yield strength.... The combination of the two requires more velocity to open the hollowpoint up, and more force to peel off the petals to form fragments.... This is the opposite of "shattering" because it is harder.... hard alloys take more velocity to expand, not less.... Note that slamming a bullet into a steel plate and having it shatter on impact is quite a different process than controlled expansion in tissue or a flexible medium....

I have done quite a few experiments with hollowpoints, and talked extensively to Erik at Hollowpoint Mold Specialties, and my experiments confirm exactly what he told me.... If the HP extends to the back of the nose, the bullet is more likely to come apart.... The shallower the HP, the less it expands for a given velocity.... The harder the alloy, the less it expands for a given velocity.... The larger the diameter of the HP, the less velocity required to initiate expansion, after which the above factors take over.... There are actually a LOT of photos of HPs made from different alloys, fired at different velocities, on his website....

If you think that using a harder alloy in airgun bullets will help the bullet fragment, you will be disappointed.... The opposite is the case, you want soft bullets to expand at our velocities.... and a deep HP cavity if you want it to come apart.... If you want it to stay together and just mushroom, you will have to find the right balance between HP cavity size and shape, alloy, and velocity.... If you have a bullet that performs "just right" and shoot it faster, it will likely at some point start to fragment.... and vice versa....

Bob

« Last Edit: March 19, 2019, 03:38:53 PM by rsterne »
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Alan

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Re: Question about fragmentation
« Reply #5 on: March 19, 2019, 04:01:44 PM »
I would agree with you, but I don't shoot bullets out of airguns. Rather, I use standard JSB Match, .25 caliber, 25.39 grain pellets. Fired at a hard backdrop at 50 yards, they just flatten. Shot at the same backstop at 10 yards, and they shatter into small fragments. I do realize we're talking about two different type of projectiles, but I still believe that if your hollow point was driven at say 1,800 fps, it would do more than just mushroom.
  • Roswell, New Mexico
Alan

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rsterne

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Re: Question about fragmentation
« Reply #6 on: March 19, 2019, 04:35:14 PM »
Velocity plus steel plate = deform, flatten, shatter in that order as you increase velocity.... Realistically, on a steel plate the shape of the pellet or bullet, HP, RN, or solid, makes little difference.... I don't pay much attention to what happens hitting a steel plate, I'm more interested in what happens on flesh and bone.... and I find that the soap is a pretty good indication of that....

Those .257 cal HPs that fragmented at 950 fps, fired at a steel plate, would completely flatten, and likely shatter or give off fragments....

Bob
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Alan

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Re: Question about fragmentation
« Reply #7 on: March 20, 2019, 04:58:37 AM »
I'm with you, Bob. Terminal ballistics is an interesting subject.

Once upon a time, I placed water-saturated, foam floral blocks, inside latex surgery socks. The idea was to have the latex act as skin, and the foam to act like animal internals. I didn't learn much from the exercise, as the .177 caliber (397) pellets always went clear through the blocks, even at 50 yards. After drying the blocks, and cutting them with a hot wire, it did show the wound channel made by the pellets, which were very similar to those seen in ballistic gel.

The one thing I haven't answered, at least for myself, is why two, otherwise identical shots, will have different results—a dead bird vs. a fly off. Figure that one out, and we'll get rich!
  • Roswell, New Mexico
Alan

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rsterne

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Re: Question about fragmentation
« Reply #8 on: March 20, 2019, 09:35:47 AM »
Birds are always a problem.... Their central nervous system is spread out along the upper part of the spinal column, and it is that area which fires the flight muscles.... Sometimes a head shot disables all communication with the flight muscles, while other times the dead bird takes flight, sometimes even under control.... for a while.... The flight muscles eventually run out of oxygen and the bird crashes....

Animals, and rodents in particular, experience "cadaveric spasms" when shot in the head.... Again, the animal is dead, but the nervous system keeps firing, triggering quite severe muscle spasms.... This often results in ground squirrels making their hole, and rabbits doing spectacular back flips.... even while missing large portions of their grey matter....

Bob
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