Bullet core stripping

G

Gene Beggs

Active member
Bullet Core Stripping

Have you ever had a batch of bullets that were super accurate but the next batch from the same maker would not shoot worth a darn? The bullet maker assures you they were made in the same die, with the same lot of jackets and cores. "Nah,, it must be something else." he says. :mad:

Before we begin our discussion, please review chapter eight in Vaughn's book, "Rifle Accuracy Facts." Harold Vaughn explains things so well. :)

With the best set of bullet dies ever made, using the best cores and jackets in the world you can still come up with bullets that will not shoot! How is this possible? :rolleyes:

There are two steps in the bullet making process that if done improperly, gaurantee poor accuracy.

We will continue with this tomorrow after you have had a chance to review chapter eight in Vaughn's book.

Later

Gene Beggs
 
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So What You Are Saying.........

is that sometimes Bullet Makers get it right, and sometimes they get it wrong.

Not very encouraging........jackie
 
jackie schmidt said:
is that sometimes Bullet Makers get it right, and sometimes they get it wrong.

Not very encouraging........jackie
Click to expand...


Jackie, the top bullet makers understand perfectly, the two errors in the bullet making process that I'm referring to and that we will discuss tomorrow. They will never be guilty of making either of them; others may not even be aware of the problem.

Later,

Gene Beggs
 
lube on the core.......acceration compression, pushes it out....core over heats melts friction...

inquiring minds want to know

mike
 
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What is meant by core stripping?

If you have studied Chapter 8 of "Rifle Accuracy Facts" (RAF) by Harold Vaughn, you have a good understanding of core stripping but for those of you who do not have a copy available I will quote here the first paragraph.

Harold Vaughn, RAF Chapter 8 page 155

"The lead core in a jacketed bullet is subjected to a large shearing stress at the interface between the jacket and the core during spin-up. As the bullet enters the rifling a large angular acceleration occurs which spins-up the jacket. The lead core is heavy and has a large spin moment of inertia that resists this large angular acceleration. The core is driven by friction forces between the core and jacket and shear stresses developed by the internal indentations in the jacket caused by the rifling engraving. These internal indentations protrude into the core about two mils. If the lead core is too weak to stand this shearing stress, core stripping results and the core will have a slower spin rate than the jacket when the bullet exits the muzzle. The maximum differential spin rate that I measured (5.5%) results in the core roll angle lagging behind the jacket roll angle by as much as 20 degrees. After muzzle exit the core slows down the jacket spin rate and the jacket speeds up the core spin rate slightly until both are the same. The resulting spin rate of the bullet is slower than it would have been if core stripping had not occurred."


Now that we understand what is meant by core stripping, lets discuss how the bullet maker can insure it does not happen.

First and perhaps most important of all is the hardness and strength of the core itself. Pure lead is very soft and pliable. As Vaughn states above, "If the lead core is too weak to withstand the shearing stress, core stripping results."

Some bullet makers use only pure lead wire in making cores, others use a lead/antimony alloy which is harder and stronger. Pure lead cores are easier to squirt and seat from the standpoint of effort on the press handle but pure lead tends to stick to the core seating punch.

Dr. Jack Jackson, editor of "RAF" by Vaughn has conducted extensive tests with different alloys to determine the optimum hardness for bullet cores. Pure lead is generally considered too soft; Linotype, which is about as hard as you can get is too hard. Somewhere in between these two extremes lies the Holy Grail.

Let's take a break and think about this for a while. If you have any questions or comments, let's hear it. Next time we will discuss why it is so important to triple wash both cores and jackets before the core seating operation. We will also discuss the importance of proper core seating pressure.

Later

Gene Beggs
 
Hey, Gene:

What sort of performance did you observe? Wild shots every now and again, or loose groups?

Harold
 
Gene,

There is also some great info about this, not as detailed, but still good in Page's book; The Accurate Rifle.

From what I have been studying / learning about from bullet makers is similar to what you are talking about - relating to lead core composition and hardness.

I don't think we use rockwell for testing lead, maybe Brinell testing? I'm not sure / still researching that.

But, I do have a question about this issue -

1. Does the type of rifling influence this? Barrels like Ratchet, 5R, ect.. Could be different as to how the influence bullet stress?

2. Would the rate of twist influence this? I would think so.

3. Land and groove depth / protrusion uniformity?

I'm curious to see where this goes.

s.
 
Stephen,
If you compose a long post in a word processing program and then copy and paste it into the composition window on this board, you will still have a copy (if you saved it) to retry with, if it doesn't post properly the first time.
 
Bullets

I am of the belief that a matrix selection for bullet cores should be much wider than only considering BHN levels. I believe in order to find the optimum bullet core alloy, factors such as the homogenous make-up, heat capacity and thermal conductance relationships have to considered.
However that being said, how much room for improvement is there? A lot has to do with quality control and, Good Jackets……………..

Ken
 
Section a 68 gr bullet and you will find that where the nose of the core
has moved into the ogive area on point up, Fold lines occure in the jacket.
these if the core was not badly damaged, appear much like the nose of a torx screwdriver. That the pressure upon firing is at the opposite end
can only tighten this keyed surface. Fold lines are more prominent than
lands marks imposed thru the jacket. I am not fond of lube lines/folds
but the may serve a purpose. In some dies, say 7 1/2 with a light core,
this may never occur, the core remaining more cylindrical. The right
core seating punch is a hugh issue. getting the right combo is tough,
as any bullet maker can attest. I can make all the core seating punches I want, and its still tough.
They core cleaning, is always the victim when thoughts of volumn or
profit come to mind. Oxidized cores are said to lock to jackets better than
fresh out of the squirt die, because of oils. I have never seen that
oxidation even on cores. Its no different than buying bad jackets. I'm
not speaking of tenths here either, more like thousands
 
Bob

Some leads naturally oxidize more than others (origin) but like as Gene mentioned many now are using Sb to harden and this alloy oxidizes very little.

Ken
 
The primary reason antimony is added to lead? To reduce the friction and decrease wear in the die. This is and industry standard, and known for this reason. It has nothing to do with this small segment of industry, we the shooting public.

Antimony gives better flow characteristics in pressure extrusions. Yes it does at a slight amount of hardness, depending on the amount in the lead antimony solution. More antimony more hardness. Lets not get this pressure extrusions mixed up with bullet casting.
 
K Hope

I am sure your correct about the oxidation. It is possible, and I think likely that what I said to be oxidation is really residue from the boiling process as
they seem to have more on the lower surfaces. as seen in a strainer while
drying. At any rate, anything but a clean dry core defeats the purpose, by
imbalencing the bullet.
 
What gives, Gene?

I'm curious to see where this thread is coming from and where it's going, but with what little indication there is, I'd have to say CLEAN jackets, CLEAN AND PROPERLY ETCHED cores, and letting the core seated jackets rest a bit before point forming would be the best insurance against core/jacket separation.
For what it's worth, my opinion is that any slippage between the core and jacket is not just going to open the group up, it will ruin the target completely.
Bryan
 
bryan said:
I'm curious to see where this thread is coming from and where it's going, but with what little indication there is, I'd have to say CLEAN jackets, CLEAN AND PROPERLY ETCHED cores, and letting the core seated jackets rest a bit before point forming would be the best insurance against core/jacket separation.
For what it's worth, my opinion is that any slippage between the core and jacket is not just going to open the group up, it will ruin the target completely.
Bryan
Click to expand...


Bryan,


Ok, I'm no expert but I HAVE fired a lot of 68gr bullets through an 8" twist. I've been advocating the 8"er as an all-round varminator for 15yrs.

IME:

Some bullets seem unaffected, for instance Nosler Ballistic Tips will group right at 1/4" minus just like they do in a 14".

Some bullets just throw the occasional "inch-out" random flyer.

And some splatter like a shotgun blast or simply puff on the way to the target.

Berger MEF's from the 80's just shoot dots at 3600fps from an 8"twist. The Match bullets will throw fliers.

All's I'm saying is, they maybe don't ALL fail or slip and a lot of what I've called "core slipping" just opens the groups up to 1/2moa with 1-3 weird fliers and the rest in a hole.

al
 
Re: core cleaning & prep.

There is another school of thought: that cores with as smooth an outside surface as possible adhere better than an etched core due to the increased surface area.
 
Very interesting thread. I'll share my 2 cents. I stopped using antimony lead 7 years ago because of the inconsistent mixing. I was getting large hardness deviations and went back to pure lead with excellent results. As for lead stripping or the melting of lead verses antimony lead, The difference in melting point is only a few degrees making this point mute. The jacket to lead bond is as I see it the most important ingredient in the bullet making mix. I don't boil my lead (This causes lead hardening with an inconsistent BHN and or Saeco reading in excess of 8 BHN) Pure lead on my Redding/Saeco hardness tester is 5 BHN or 0 Saeco. I use a chemical solution to etch my lead cores. As for lead build up on the core seat punch I tend to believe it is the Copper hardness that causes this. Have only seen 2 lots of jackets that this is a problem.

JDS Bullets
 
Good point, Al

But with the tons of pressure used in core seating, the outside of the core should be a mirror image of the inside of the jacket, regardless of the surface characteristics of either.
Bryan

I'm just a "used to be" bullet maker..............my guess is that the current crop of bullet makers will refrain from making posts till Gene gets more specific. Easier to justify the responses, ya know.
 
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Core stripping continued,,,

In post number one, I stated,

"There are two steps in the bullet making process that if done improperly, gaurantee poor accuracy."

They are:

1. Proper degreasing of cores before seating.

2. Seating cores with the correct pressure.


Why is it so important to thoroughly degrease cores before seating? Because the slightest trace of lube reduces friction between the jacket and core increasing the likelihood of core stripping.

There are many solvents that can be used to degrease cores and most bullet makers have their own preferences, but the most effective agent I have found is Trichlorethane, which is a hazardous material and must be used with caution. It's hard to find and I don't use it since Dow Chemical introduced their NuTri which seems to work just as well but is safer. NuTri is expensive but well worth it and evaporates quickly especially if you heat the cores with a heat gun. It removes all traces of oil, grease and other contaminants and leaves no residue. One should keep three containers of solvent and rotate them so the last bath is always pure and clean. The first bath removes most of the lube and will quickly turn dark. The second bath removes virtually all contamination and the third bath insures that the cores are squeaky clean.


Why is core seating pressure so critical? It's all about achieving the tightest grip (maximum friction) between the jacket and core. If the cores are not seated hard enough the jacket will not get a good grip on the core; if seated too hard, the jacket will be stretched beyond the yield point weakening its grip on the core.

A very important thing to remember; lead is a dead material and has no springback, while the gilding metal jacket does. During core seating, the jacket and lead core are expanded together. When the core seating punch is withdrawn, the lead core remains fully expanded while the jacket springs back, creating a very tight grip on the core.

We will continue our discussion tomorrow. Hope this helps.

Later,

Gen Beggs
 
It finally makes sense... why threads like "my .270 won't shoot need help" shouldn't be on this board. I always knew the surface of the meaning of this board, but this just solidifies it. I am in awe of the knowledge and experience here.
Everytime I feel like I have advanced to a new level of understanding I am shown that I don't have a CLUE about this sport.
this and the gain twist info is great winter reading.
Just wanted to throw that in coming from a man who is still learning,
Mike
 
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