Does the 1:7 twist require harder alloy?

[threehundred]

Everytime I come upon this thread, I want to go through my files and pull up a great article I believe says a lot on this subject, but, I never do, until now. Sorry for resurrecting an old thread, but I think obturation and the balance of lube, lead hardness and obturation for the prevention of leading, is a lot more simple than I have seen presented by many folks.

My apologies to anyone that knows of these facts or this article and may be bored by its repetition.
I just think there are a lot of misconceptions on shooting soft lead, I know I had a few many years ago.

http://www.lasc.us/Fryxell_Book_Chapter ... llurgy.htm

Here is a small taste of the article which is a long read, but I always find something I previously missed, nearly every time I read it.

"Obturation is also supported by the sealing effects of the bullet lubricant (see lube chapter). In the absence of obturation, the entire burden of sealing the bullet/bore interface falls on the lube. With a top-notch lube this can be accomplished, but building teamwork between the alloy and the lube is a better way to do things. Is obturation necessary for good cast bullet performance? No. But it IS a tool that we can make use of and make work for us, so why not take advantage of it?

Hardness. So we want to make sure that a bullet isn’t too soft, or leading will result through galling and abrasion, and we want to make sure that it isn’t too hard so we don’t lose the beneficial effects of obturation, and fall prey to leading through gas-cutting. Does that mean that we have to hit a very specific hardness for each cast bullet application? Thankfully, the answer to that question is “No”. Rather, there are a range of hardness's that serve very well for each pressure/velocity level.

Useful
Application Hardness Range
Light target loads (<800 fps and 10,000 psi) BHN 6-12
Standard revolver loads (800-1000 fps, 16,000 psi) BHN 8-14
+P revolver loads (1000-1200 fps, 20,000 psi) BHN 10-16
Magnum revolver loads (1200-1500 fps, 35,000 psi) BHN 12-20
454 Casull (1400-1800 fps, 50,000 psi) BHN 16 and up
The lower end of each of these hardness ranges will expand somewhat in each of these applications. Harder bullets can be used, but they won’t obturate meaning that you’ll have to use a lube capable of sealing the system, since the bullet cannot contribute to this critical job. Hard lubes probably won’t work here. Note the recurrence of BHN 12 in many of these ranges, and remember that’s what the Oldtimers used to think of as a hard bullet. We’ll come back to this thought…"

[dellet]

Everytime I come upon this thread, I want to go through my files and pull up a great article I believe says a lot on this subject, but, I never do, until now. Sorry for resurrecting an old thread, but I think obturation and the balance of lube, lead hardness and obturation for the prevention of leading, is a lot more simple than I have seen presented by many folks.

My apologies to anyone that knows of these facts or this article and may be bored by its repetition.
I just think there are a lot of misconceptions on shooting soft lead, I know I had a few many years ago.

http://www.lasc.us/Fryxell_Book_Chapter ... llurgy.htm

Here is a small taste of the article, it is a long read, but I always find something I previously missed, every time I read it.

"Obturation is also supported by the sealing effects of the bullet lubricant (see lube chapter). In the absence of obturation, the entire burden of sealing the bullet/bore interface falls on the lube. With a top-notch lube this can be accomplished, but building teamwork between the alloy and the lube is a better way to do things. Is obturation necessary for good cast bullet performance? No. But it IS a tool that we can make use of and make work for us, so why not take advantage of it?

Hardness. So we want to make sure that a bullet isn’t too soft, or leading will result through galling and abrasion, and we want to make sure that it isn’t too hard so we don’t lose the beneficial effects of obturation, and fall prey to leading through gas-cutting. Does that mean that we have to hit a very specific hardness for each cast bullet application? Thankfully, the answer to that question is “No”. Rather, there are a range of hardness's that serve very well for each pressure/velocity level.

Useful
Application Hardness Range
Light target loads (<800 fps and 10,000 psi) BHN 6-12
Standard revolver loads (800-1000 fps, 16,000 psi) BHN 8-14
+P revolver loads (1000-1200 fps, 20,000 psi) BHN 10-16
Magnum revolver loads (1200-1500 fps, 35,000 psi) BHN 12-20
454 Casull (1400-1800 fps, 50,000 psi) BHN 16 and up
The lower end of each of these hardness ranges will expand somewhat in each of these applications. Harder bullets can be used, but they won’t obturate meaning that you’ll have to use a lube capable of sealing the system, since the bullet cannot contribute to this critical job. Hard lubes probably won’t work here. Note the recurrence of BHN 12 in many of these ranges, and remember that’s what the Oldtimers used to think of as a hard bullet. We’ll come back to this thought…"

That's a very interesting read and it applies very well to subs in a Blackout, not only for cast but also for copper and brass solids.

Depending on the platform and powder combination getting pressures anywhere from 17-55,000 psi is possible.

Copper solids need about 45,000 to expand the base so most sub loads end up with gas blowing by the bullet.

[Rooter pig]

I always thought the cast bullet sealed the bore by being at least bore size or bigger. Seems to me they don't need to obturate if they already seal. Maybe I'm missing something.

[threehundred]

Article quote;

"Obturation. OK, if we know that soft bullets with a BHN of 6 can cause problems, why don’t we just cast everything out of linotype? If a little hardness is good, then more is obviously better, right? Well, aside from being a really expensive way to make cast bullets, there are some physical drawbacks to this approach. Obturation is the plastic deformation of the bullet metal in response to the applied pressure (from the burning powder). Cast bullet obturation was extensively studied and characterized by Dr. Franklin Mann over a century ago, and summarized in his most excellent treatise The Bullet's Flight from Powder to Target. Using soft cast bullets, he observed bullet swelling from several thousandths of an inch to several calibers, depending on the conditions employed (pressure, barrel condition, etc.). Modern barrels are exceptionally well-made, but there are minor imperfections (one or two ten-thousandths of an inch) in groove diameter, the width of the lands and grooves, minor local variations in twist rate, etc. As the bullet is engraved, these minute imperfections result in an imperfect seal between the bullet and the bore. The defects in this seal will be the same size as the variation in the dimensions. Since the hot gas molecules that are driving the bullet down the bore are less than one ten thousandth this size, gas leakage is a problem. A lot of attention has been paid to groove diameter and hand-lapping or fire-lapping to make this diameter more uniform through the length of the bore. Another issue that is also addressed by such lapping is that of the grooves and lands. If the grooves and lands vary in width, then this seal also is compromised. The forward edge of the land isn’t so much of an issue because the bullet’s forward momentum continuously drives it into this edge, forcing this seal closed. It’s the trailing edge where the seal is compromised if the dimensions vary. This is why it’s not uncommon to see leading “follow the rifling”, the trailing edge seal was compromised and the gas-leak cut the bullet metal at this point and deposited the metal fouling at its point of generation. By matching the bullet hardness to the pressure of the load, we can exploit obturation to prevent this problematic fouling. By reacting to the applied pressure, the bullet metal can undergo plastic deformation to conform itself to the local profile of the barrel, and help to maintain the seal.

It is important to recognize that obturation is not simply an increase in bullet diameter, it is also a backfilling of defects obtained in the engraving process, and therefore plays a role in every shot fired with a cast bullet, even those that are properly (or over-) sized for the bore.

Some folks don‘t like to believe that obturation plays an important role in cast bullet performance. These “naysayers” like to point out that this mechanism only operates at the peak pressure of the load, which only applies to a short period of time and a small stretch of the barrel. This is not true. The models and correlations that experimental ballistician’s have put together to explain the observed behavior generally tend to correlate peak pressure to bullet hardness. This is simply the model that we use to explain the observed data. All metal undergoes some response to applied pressure, the magnitude and speed of that response depend heavily on the hardness of the metal, but lead alloys are soft and the degree of deformation is proportional to the applied pressure. It is important to also note that the rate of gas leakage (and hence gas-cutting) is also a direct function of applied pressure. Thus, peak pressure induces the most and fastest obturation, and enhances the bullet/bore seal when it is needed most, at peak pressure. Lesser pressures at other points along the P vs. T curve induce smaller (and slower) degrees of obturation, that still play a role in maintaining this seal. Obturation is not an on-off switch that only operates at peak pressure, that is simply how the models that have been applied to explain it work.

Obturation is also supported by the sealing effects of the bullet lubricant (see lube chapter). In the absence of obturation, the entire burden of sealing the bullet/bore interface falls on the lube. With a top-notch lube this can be accomplished, but building teamwork between the alloy and the lube is a better way to do things. Is obturation necessary for good cast bullet performance? No. But it IS a tool that we can make use of and make work for us, so why not take advantage of it"