Cast bullet accuracy and trailing edge failure

[popper]

F/A = .00000697 · ((WrV²) / (Pldn)) I did the calc for my 150 gr in the AR308. 4389 psi? Very close to my calc a few years ago, for the rifling engaging psi. Once the boolit is spinning, force drops by x10. Couple years ago I did some 'twist till break' tests on HT #2 30 cal rifle (RD mould). I could apply ~ 30# with 6" vise grips and get a breaking force of ~360#, ~4741 psi. The weak point is a the lube grooves (smaller dia), so this is a good formula. I posted a psi number (435 psi) for softening of HT Pb62%Sn, which is close to HT #2 alloy. Note that number is NOT for fracture, just the force applied to a disk used to measure the BHN (Hv - vickers hardness) of the alloy. The Hv at the disk center remained ~10, IIRC ~ 3 at the edge, indicating that the alloy softening occurs primarily at the outside of our boolits AND doesn't recover while moving down the barrel. We wonder why we get gas cutting? It also adds some validity to Larry G's twist vs accuracy discussion. Also adds validity for Cu jackets & to some extent, PCing.
A flat nose boolit is less stable than a round nose of the same length primarily because the FN slows down faster and enters the transonic region earlier (~mach 1.5-0.8).

[Ricochet]

I'm presently reading Mann's "The Bullet's Flight...", only to page 127 I think so far. I found interesting his experiments with extremely short barrels so that the pressure at exit was higher than the obturating pressure of the lead, producing impressive bullet expansion from the rear. His brother named this the Putty Plug Principle, meaning that the bullet in the bore under pressure was like a plug of putty, confined by the bore walls to keep it cylindrical. He found the effects of base damage to be minimal, but he was always using wads. He also found that deliberately damaging the muzzle in quite severe, asymmetrical fashion shifted groups with relation to point of aim without affecting group size. I think a takeaway is that wads and fillers were widely used in the 19th century for good reason with lead bullets. We probably should do likewise.

[geargnasher]

F/A = .00000697 · ((WrV²) / (Pldn)) I did the calc for my 150 gr in the AR308. 4389 psi? Very close to my calc a few years ago, for the rifling engaging psi. Once the boolit is spinning, force drops by x10. Couple years ago I did some 'twist till break' tests on HT #2 30 cal rifle (RD mould). I could apply ~ 30# with 6" vise grips and get a breaking force of ~360#, ~4741 psi. The weak point is a the lube grooves (smaller dia), so this is a good formula. I posted a psi number (435 psi) for softening of HT Pb62%Sn, which is close to HT #2 alloy. Note that number is NOT for fracture, just the force applied to a disk used to measure the BHN (Hv - vickers hardness) of the alloy. The Hv at the disk center remained ~10, IIRC ~ 3 at the edge, indicating that the alloy softening occurs primarily at the outside of our boolits AND doesn't recover while moving down the barrel. We wonder why we get gas cutting? It also adds some validity to Larry G's twist vs accuracy discussion. Also adds validity for Cu jackets & to some extent, PCing.
A flat nose boolit is less stable than a round nose of the same length primarily because the FN slows down faster and enters the transonic region earlier (~mach 1.5-0.8).

I maintain that twist vs. velocity is a matter of bullet balance. Past a certain RPM the imbalances imparted to the bullet through NORMAL casting and loading techniques really sling the bullets out of the group. Most of that imbalance is probably imparted during the first inch of the firing event. Not sure how or if any work-softening of the driving bands during the engraving event affects high-velocity accuracy, but I suppose it could contribute to obturation failure at the trailing edge of the lands if the alloy is softening.

The results of my calculations in several rifles and handguns is surprising to me, and interestingly I've found that with my limited data is a positive trend to groups opening up when the leading edge pressure per Mann et al exceeds the Lee formula for chamber pressure vs. Brinnell hardness number of the alloy. I'm thinking with a little more data we might be able to note a valid trend here, where chamber pressure is ignored but only the rifling stress pressure vs. bhn is taken into account. To put that statement in perspective, one rifle load of mine that is one grain above the system's accuracy threshold has an estimated peak chamber pressure of just under three times the maximum recommended by Lee, but only a single-digit percentage above what I'll call the "land stress maximum" for the alloy. Below the land stress maximum groups were very good. This of course doesn't take into account any work-softening of the alloy when fired.

I've been told by a couple of people who ought to know to ignore rifling stress, it's insignificant. After doing the math and comparing calculated land pressure to a few groups with various loads, I'm not so sure that calculating land pressure and taking that into consideration when alloying and loading is a waste of time, it may not be. Take all this FWIW.

Gear

[Forgetful]

Any way to case harden lead so you have a softer core?

[geargnasher]

Paper or copper jackets.

Gear

[popper]

Gear - I was interested in the boolit entering rifling torque part of the equation, slower twist, lower torque on the boolit. Like trying to screw a wood screw into steel hole. They always twist off.
rifling stress pressure vs. bhn is taken into account Darn now I have to check my old Xcel calcs for rifling force on the boolit going down the barrel. I used some pressure data from Larry G. to run all the calcs, but I didn't look at that one closely.

[303Guy]

Flat nose boolits are less stable than round nose because they undergo higher tipping forces due to their shape. As they slow down their stability increases because the tipping forces diminish. Tipping forces always increase with velocity increase but with spitzers that increase is a little less than the increase in spin stability to a point.

[SniderBoomer]

I Paper-Jacket my 45-70 boolits a lot, this is the base of a recovery, the paper was cut neatly at the base edge, not folded over. The load used no filler, the lead used was roofing lead and tested to BHN 5.

Is the lack of filler the reason the powder indents are consistently on one half of the base like this?

To be honest, these are accurate as heck in my 1895, but if I can improve things some, why not.

[Larry Gibson]

From post #163 above;

"I maintain that twist vs. velocity is amatter of bullet balance. Past a certain RPM the imbalances imparted to thebullet through NORMAL casting and loading techniques really sling the bullets out of the group."

and......

From; http://castboolits.gunloads.com/show...velocity-chart


"The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent."

.............is there a difference between the two?

Larry Gibson

[btroj]

Deleted

[Bullshop]

From post #163 above;

"I maintain that twist vs. velocity is amatter of bullet balance. Past a certain RPM the imbalances imparted to thebullet through NORMAL casting and loading techniques really sling the bullets out of the group."

and......

From; http://castboolits.gunloads.com/show...velocity-chart


"The RPM threshold is that point where accuracy begins to deteriorate when the RPM is sufficient to act on imbalances in the bullet in flight to the extent the bullet begins a helical arc in flight or it’s flight path goes off on a tangent."

.............is there a difference between the two?

Larry Gibson

Larry
I brought this up in another thread about high velocity with cast boolits but no one wanted to touch it.
If as you say the twist vs velocity is a matter of boolit balance then I have to wonder why I can get good accuracy with a 55gn .224" boolit fired in a 1/10" twist at over 4000 fps when used in a sabot but cant get good accuracy in the same twist barrel and boolit when fired in a .224 cal barrel.
It seems that if balance was the factor causing the difference why is one balanced and one not? What is it that is causing the imbalance in the boolit fired from the right caliber barrel and not in the one fired from an over caliber barrel in a sabot?
The torque of the boolit being screwed into the barrel has to be causing the imbalance if imbalance is the cause of inaccuracy.
It cant be RPM alone because shooting the same boolits at extreme rpm using a sabot is accurate.

[geargnasher]

Bullshop, the sabot works the same way as a paper or copper jacket: By protecting the core from damage due to crooked start or, if it is even a cause, the rifling. Balanced bullets tend to shoot very straight, very fast, and are pretty much immune to RPM.

Gear

[Larry Gibson]

Bullshop

Your question has been discussed and answered in several other threads on the subject. Post #172 is an excellent synopsis of the answer.

Larry Gibson

[Bullshop]

Thank you

[leftiye]

Larry
I brought this up in another thread about high velocity with cast boolits but no one wanted to touch it.
If as you say the twist vs velocity is a matter of boolit balance then I have to wonder why I can get good accuracy with a 55gn .224" boolit fired in a 1/10" twist at over 4000 fps when used in a sabot but cant get good accuracy in the same twist barrel and boolit when fired in a .224 cal barrel.
It seems that if balance was the factor causing the difference why is one balanced and one not? What is it that is causing the imbalance in the boolit fired from the right caliber barrel and not in the one fired from an over caliber barrel in a sabot?
The torque of the boolit being screwed into the barrel has to be causing the imbalance if imbalance is the cause of inaccuracy.
It cant be RPM alone because shooting the same boolits at extreme rpm using a sabot is accurate.

I see where you are coming from. I agree that if there wasn't the factor of damage to the boolit that the .22 boolit shot in the .22 barrel would be the more accurate as the sabot would center and balance the boolit less well. Balance goes out the window along with straight starting, and correct launch with damaged boolits.

[303Guy]

Here the trailing edge failure is quite evident. This is from boolit swaging.

[s mac]

Here the trailing edge failure is quite evident. This is from boolit swaging.

By swaging do you mean an oversize boolit? Swaged in the bore?

[Bullshop]

I see where you are coming from. I agree that if there wasn't the factor of damage to the boolit that the .22 boolit shot in the .22 barrel would be the more accurate as the sabot would center and balance the boolit less well. Balance goes out the window along with straight starting, and correct launch with damaged boolits.

Then breach seating or muzzle loading the boolit as was being done by target shooters at the beginning of the 20th century should completely eliminate any damage being done to the boolit upon acceleration and transition from case neck to being fully into the barrel.
Those same target shooters eventually went to fixed ammo as being equally as accurate as the former two methods of loading a breach loading target rifle if and when every precaution was addressed to achieve perfect alignment of boolit to bore.
Since the loads that were being used at that time were pretty much limited to black powder type pressures the torque being applied to the boolit from a state of rest to a spinning projectile was reasonably low compared to some of the pressures we load to now.
I submit that the torqueing of the boolit is a cause of damage to the boolit even when the alignment of boolit to bore is perfect.
Think of it like you were trying to turn a lead bolt with a steel wrench. The greater the torque the greater the chance the bolt will break. Sloppy chamber dimenti9ons would seem to fall into place with this idea as well as harder alloys for higher pressure, torque.
It also seems that longer boolits transitioning from case neck to fully inside the barrel may suffer internal structural damage from this torqueing at different interval for the length of the boolit. As the boolit is moving forward and beginning to take on the torque stresses at its nose the base end is not yet affected by the torque of the rifling trying to get the boolit spinning. I believe this can or may cause internal structural damage at a micro level. Greater pressure would give higher velocity which would cause greater torque pressure on that lead bolt example I mentioned.
Smack your steel wrench on the lead bolt with a big hammer and the bolt will break. Apply slow steady pressure within the strength limits of that bolts make up and it may just turn before it breaks.
Evidence of this uneven torque for the length of a boolit is evidenced by the uneven rifling engraving that can be seen on recovered boolits, especially revolver boolits. You will most often see that the engraving is wider at the lead drive band than at the base showing that there was some torque being applied to the nose before it was applied to the base.
A rifle chamber has some amount of unsupported free travel the boolit must transition to be fully inside the barrel just as does a revolver so is torqueing in the same manner. Is this not why we always see repeated that "FIT IS KING" because a tight fit would allow less damage to the boolit as the torque is being applied. The tight fit would help to contain the amount the boolit can deform just as does a breach seated boolit.
I can not deny that alignment is an absolute critical issue but I also submit that there is more to the equation than alignment alone. I also submit that high rpm alone is not the sole reason for imbalanced boolits shooting erratically but that the higher rpm (torque) is actually a cause for the damage causing the imbalance. Again this idea is evidenced by the fact that as load pressure is increased and so torque and rpm a harder alloy is needed.
Maybe I am off base and am sure someone will tell me so and exactly why but that is my reasoning and the way I see things now.

[303Guy]

By swaging do you mean an oversize boolit? Swaged in the bore?

Yes, swaging in the bore with a throat fitting boolit which is oversize for the bore.

[popper]

Thanks Bullshop. I would love to do tests with different alloy using a multi-ton high speed pneumatic press to see what the failure mode of our boolits is. I do think the 'oversizing' of our boolits could be stidied also. I can agree with L.G.'s twist theory but I do think the kick-in-the-seat vs twist has a lot do do with accuracy. Now I'm wondering if PP actually helps launch.