The Relationship of Center of Gravity and Pressure with Slugs?

[Ranch Dog]

This is from another thread on another topic...

Now I am wondering... shouldn't center of pressure be behind the CG for drag stabilized slugs and in front for slugs from rifled guns?

From what I've seen of seen of the slow-motion videos, that hasn't been working out very well for the Lee slugs as their center of pressure is definately behind the center of gravity. I think for any projectile to be successful down range, a repeatable MOA of any size, the center of gravity needs to be slightly aft of center as well of the center of pressure. From that, the center of pressure needs to be slightly ahead of the center of gravity.

The problem for most is measuring either value. The math is tough and the only user-friendly software that can do the math, as far as I know, is TMT's Cast Bullet Design. Even that program has problems with a tapered skirt with a hollow base. Same with the actual "key" within the hollow base of the Lee. Still, I think the calculated measurement is very close. The TMT software measures the values from the base of the projectile expressed in a percent of the total bullet length. As a general rule, I like seeing a CG about 47.0 to 48.5% from base, with the CP greater, and within 1% of the calculated CG. This is simply a comparison on my part, the numbers from the drawing against MOA performance downrange.

Have you considered round balls? 0.678" RB's in an appropriate shotcup are a good fit. 0.662" RB's in steel shot shotcups are a good fit. and 0.735" RB's are a good fit for bore size. Even 0.690" RB's in thin petal shot cups could work.

I haven't to date as the rules for the hunt specifically state "slugs." I'm not a fan of round balls for hunting, I've shot a bunch of small game with my centerfires, 218 Bee through 38-55 Win, using alloys from pure lead to Lyman #2, and never have I cleanly dispatched game, not once. I am going to pursue round balls through my shotgun shooting this year, just to see how they perform in smooth vs. rifled barrels.

Theoretically, the CG and CP are centered in the round ball. I've always heard to put the sprue cut up front, but what makes sense to me it to put it to the back. That flat represents missing material which would shift the CG slightly aft, leaving the CP at 50%.

Of course, all this is just armchairing. I'm one that likes burning powder even if it doesn't go as planned.

[megasupermagnum]

You've never dispatched small game cleanly? I've cleanly killed small game with .177 bb guns and blowguns, although the blowgun dart had a broadhead.

The problem with putting the sprue down, is that you can't see it to align. You could eyeball it, and put a sharpie dot on the other side, I choose the easy route and put sprue up. As slug shooters, we have a lot more to deal with than just bullet fit and balance. Slugs that shoot good at 75 yards are often wild fliers by 150. Slugs that have the balance to fly to 200 yards, often don't shoot good.

[Ranch Dog]

You've never dispatched small game cleanly?

Only with head shots.

[Mr_Sheesh]

CP has to be behind CG or your bullet is dynamically unstable. Same thing applies for aircraft and rockets (model or full size); Look at a rocket and you see larger fins on the back of them, to bring CP backwards.

Think of it as "the drag pulls back on the bullet from the CP, and the inertia pushes forwards from the CG" and you can see why. If the CG were behind it'd happily swap ends with the CP, which would mess your accuracy up horribly.

Model Rocketry handbooks or "learn to be a pilot" handbooks should cover this pretty well as it's one of the fundamentals of either field.

[longbow]

Ranch Dog:

There are round balls and there are round balls! A 12 ga. round ball is a substantial projectile. A WW 0.735" RB weighs about 575 grs. and is of course LARGE!

Not sure if "slug" is defined as a standard Foster HB, solid slug, Brenneke or whether it just refers to a single "bore" diameter projectile from a shotgun but I doubt many living things would walk away after being shot with a hard cast 12 ga. ball at 1400/1500 FPS!

Longbow

[megasupermagnum]

Ranch Dog:

There are round balls and there are round balls! A 12 ga. round ball is a substantial projectile. A WW 0.735" RB weighs about 575 grs. and is of course LARGE!

Not sure if "slug" is defined as a standard Foster HB, solid slug, Brenneke or whether it just refers to a single "bore" diameter projectile from a shotgun but I doubt many living things would walk away after being shot with a hard cast 12 ga. ball at 1400/1500 FPS!

Longbow

I showed some guys the .735" round ball. They thought they were small cannon balls! You could cause damage to someone just throwing them by hand.

One thing I forgot to bring up, they don't come out of the end of the gun round. They end up oval shaped, with a 3/16"-1/4" band that's engraved by rifling. I wouldn't worry which end the sprue is on at that point. Also, I've found my best luck with a scoop of PSB buffer under the balls. Recovered slugs show a cool pattern on the buffer side. Groups are astounding at 50 yards, but I'm still trying to find that 100+ yard load. So far I average in the 6" range at 100 yards and the round ball. If I could figure out why one side engraves more than the other, and solve it, I bet groups would improve dramatically.

[W.R.Buchanan]

Guys, Sheesh has it right. Also after dropping some of my slugs many times they always hit nose first meaning the CG is definitely forward.

Also LB's point about walking away from a Round Ball hit is spot on, and you can consult with the British Empire for the results of that, as the Brown Bess Musket was standard issue for over 100 years (1722 -1836) and more. Getting hit by a .75 cal round ball would definitely ruin your day.This gun was made obsolete by the Percussion Cap and then further by the self contained cartridge.

BUT!!! A 12 ga. Shotgun is essentially a Breech Loading Repeating Musket using self contained rounds. (the term "Rounds" came from Round Balls.) Many things are different but the payload can easily be the same as 300 years ago and it is just as effective today as then. It's just easier to clean up after. It lives on due to the variety of stuff you can shoot out of it.

If you go one farther and look at these Lyman Slugs you see basically a Wad Cutter and as such the energy transfer of the slug on the target is instantaneous. I don't see super penetration, but I do see knockdown power.

More numbers. I did some ftlb Calcs today versus TKO numbers.

Lyman Slug at 1300 fps 1877 ftlb or just under a .243 but the TKO is 67. TKO of the .243 is 11

A 405 gr .45-70 at 1600 fps is only TKO 42 but 2300 ftlb.

Lyman Slug at 1500 fps, 2500 ftlb with TKO of 78 where as a .458/500 Win Mag is 5100 ftlb but a TKO of only 70.

Federal 1 1/8 oz slugs at 1610 fps are 2875 ftlb with a TKO of 83 .30-06/180 is 2800 ftlb but TKO is only 21

What the TKO is showing is the relative energy transfer of the projectile to the target. Where as the FT LB is showing the available energy.

Used to be that the method of figuring ft lbs was to shoot a hanging steel plate and see how far it moved. This method extracted all the energy of the bullet on impact. But I wonder if the results would be more like the TKO numbers as opposed to ftlb numbers?

The difference is that the smaller dia projectile doesn't transfer it's energy to the soft target as efficiently as the larger diameter slug does. So it becomes the old Porsche doing 150 mph versus the Bus doing 90 comparison. The bus has a larger frontal area, therefore it will transfer more of it's energy to you faster than the Porsche will. Both will kill you but one will squash you flat in the process.

That is "knock down power." Something to think about.

Randy

[longbow]

MSM:

I've found the same thing in my smoothbore. Generally the 0.735" RB's show an uneven "belt" around the middle. I have thought it was due to one or two things... or maybe some of both:

- the ball leaving the hull a bit off center then smacking the forcing cone so rolling into the bore
- the ball picking up a spin as it opens the crimp so it is rotating when it hits the bore... skidding to a stop (same net effect as above)

I strongly suspect that if a hull fit full length into a chamber when fired and there was a more abrupt leade like in a rifle we would not see the uneven "belt". The ball would be driven past the crimp and directly into the bore/rifling with little to no chance of picking up spin.

However, I digress again... this thread is about CG and CP.

My limited understanding of this is that with rifle and handgun boolits (or bullets) the CP should be forward of the CG for stability (think jacketed HP target bullets) where I am pretty sure for a drag stabilized slug the CP should be aft of the CG.

I am not actually sure how the physics works with a round ball. One would initially think that the CG and CP would both be at the center of the RB but... once there is a shockwave in front and drag behind is that true?

Does the CP shift due to the shockwave? If so I'd suspect it shifts forward and maybe that is why RB's pick up a spin at longer distance... or at least I suspect they pick up a spin explaining their curve ball trajectories at longer ranges.

Is CP due to geometry or is it a dynamic thing that is affected by air pressure/shockwave?

Some smart guy out there must know these things. I am a simple country bumpkin so without doing a bunch of research I don't have an answer.

To add to the confusion turbo1889 posted that the Lyman sabot slug was not stable out of smoothbore after it went transonic, that it would tumble. Yet I have read posts where people claim very good accuracy out to 100 yards from smoothbore. I'd be surprised it they were still supersonic at 100 yards.

Now we have at least two people I will believe if they post their results using the new NOE Lyman clone. Not sure if Ranch Dog has a smoothbore to test them in but Randy certainly does.

I would like to buy one of the NOE moulds and probably should but not only will it cost me about $200 CDN by the time I have it, turbo convinced me several years ago that slugs cast from my home made moulds will shoot as well. I passed on the Mihec Lyman clone that time. Maybe yes, maybe no but I'm off work again so will leave a mould purchase until another time if they prove to be good shooters by people I trust.

For anyone with a Lyman sabot slug mould or clone, lease post results from both rifled and smoothbore guns.

Longbow

[Ranch Dog]

CP has to be behind CG or your bullet is dynamically unstable. Same thing applies for aircraft and rockets (model or full size); Look at a rocket and you see larger fins on the back of them, to bring CP backwards.

Think of it as "the drag pulls back on the bullet from the CP, and the inertia pushes forwards from the CG" and you can see why. If the CG were behind it'd happily swap ends with the CP, which would mess your accuracy up horribly.

Model Rocketry handbooks or "learn to be a pilot" handbooks should cover this pretty well as it's one of the fundamentals of either field.

I agree with you on aircraft and rockets from my experience as a 31-year airline pilot. Aircraft have wings and rockets have propulsion. Bullets are different, the center of pressure is always ahead of the center of gravity as all they have is the effects of external ballistics causing the pressure against their shape. It's the same with an artillery projectile. I do have several hundred factory bullet drawings that confirm the positions of the two references plus about twice as many cast bullet drawings, bullets that shoot great. The hollow base bullets designs I've worked with still have the CP ahead of the GG. The only place I've seen the reference positions reversed is with the hollow base slugs and the pellets used in airguns.

From Long Range Shooting: External Ballistics - Bullet Shape, here is the short of it.

"Therefore, because of the bullet shape, the projectile will always fly at an angle, called angle of attack, relative to the trajectory. ... Since the point of pressure is positioned in front of the center of mass, the air flow will force the bullet to rotate backward, pivoting on its center of mass."

Ranch Dog:

There are round balls and there are round balls! A 12 ga. round ball is a substantial projectile. A WW 0.735" RB weighs about 575 grs. and is of course LARGE!

Not sure if "slug" is defined as a standard Foster HB, solid slug, Brenneke or whether it just refers to a single "bore" diameter projectile from a shotgun but I doubt many living things would walk away after being shot with a hard cast 12 ga. ball at 1400/1500 FPS!

Very true longbow, a 12 gauge slug is not your thirty-something caliber roundball. Like I said, I'm going to try them to see how they shoot and will eventually send one towards a hog, that easy here in South Texas.

As far as the Refuge hunt goes, I'm not going to battle the feds over whether a round ball is a slug or vs. versa. I'm really in a fix over the hunt anyway. I'm supposed to leave Thursday morning, but my 90-year-old dad is having some issues. He lives here on my ranch in a separate house. Unfortunately, my wife left yesterday to go to PA to visit her sister who is having health issues. I would hate to miss the hunt after all this prep, but of course, my dad would come first.

[longbow]

Sorry to hear about your dad and sister in law. Tough times when family is ailing. Bad timing but family comes first for sure.

I hope all works out for the best and health issues are taken care of.

Take care.

Longbow

[W.R.Buchanan]

I must have a misunderstanding of Center of Pressure as to my mind the CP is at the rear end of the bullet or where the push is coming from. How is Center of Pressure established?

Also the description of angle of attack seems incorrect as a Bullet's Axis of Rotation does in fact follow the trajectory as long as the amount of spin is adequate to keep it stabilized in flight. When that spin is no longer adequate to stabilize the bullet it starts to Yaw and that Yaw will be around the Center of Gravity of the bullet with the front end spiraling around the Axis of whatever rotation is left. In other words the bullet will keyhole. But with multiple shots on the same target the keyholes won't all be going the same way so that point of spin degradation will be random. Eventually if shot out past the point where the spin is no longer affecting the flight the bullet will fall strait down sideways as the air pressure equalizes around the the length of the bullet. Like if you shot it off the rim of the Grand Canyon, the bullet wouldn't reach the other side and simply fall to the bottom and should hit the ground sideways. If you drop a rifle bullet, it will fall flat.

Our Lyman Slugs however will hit nose first. If dropped sideways from as low as 3-4 feet, they have enough weight forward to still hit nose first. I did it a bunch of times today and it hit nose first every time.

Am I FOS here?

Randy

[pashiner]

I believe that the illustration in ranch dog's post depicts an over-stabilized bullet with it's nose-up attitude in reference to the trajectory. When this happens, the gyroscopic forces from excessive spin broaden the angle of attack. I wonder what that does to B.C. , and if it effectively moves the center of pressure?

[Tom Myers]

I must have a misunderstanding of Center of Pressure as to my mind the CP is at the rear end of the bullet or where the push is coming from. How is Center of Pressure established?
Randy

Randy,
To put it simply,

Center of Gravity is the balance point of an actual bullet. Gravity, inertia and rotational forces act upon the center of mass of the bullet.
Center of pressure is the balance point of a bullet profile. Air pressure acts upon the geographical center of the profile of the bullet.

The Center of Gravity is calculated by averaging the distance from the base of the bullet to the center of gravity of each segment of the bullet.
The Center of Pressure is calculated by averaging the distance from the base of the bullet to the geographical center of a profile of each segment of the bullet.

You can find the approximate Center of Gravity by balancing the actual bullet on a knife edge.
You can find the approximate Center of Pressure by balancing a scale cardboard cutout of the bullet on a knife edge.

The pressure is created upon the lower side of the bullet profile by air pressure as the bullet travels the length of the trajectory with a slightly nose up attitude.

Hope this helps.

[Ranch Dog]

Our Lyman Slugs however will hit nose first. If dropped sideways from as low as 3-4 feet, they have enough weight forward to hit nose still first. I did it a bunch of times today, and it hit nose first every time.

That is a perfect explanation of why they don't shoot well, the CG is too far forward (greater than 50% from base). I just held a .460" 525-grain bullet horizontally and dropped it from the same distance, and it hits the ground horizontal. I dropped it from 10', it still hits horizontal. CP isn't involved with a dead drop.

There is no magic why outfits like Lightfield produce slugs that perform like rifle bullets; they are slugs and not saboted rifle bullets, they've paid attention to the principles behind interior and exterior ballistics. If your need is short range, then you have what you need in a hollow base design like the Lee. I purposely have not included the Lyman with the Lee, as I have not shot it.

pashiner, the illustration is just that. The reference points, trajectory, and angles are exaggerated for example. The tolerances of all this are rather tight in reality. I've designed about 50 bullets that are in use and have had a couple fail because I push a bit on the CG and CP positioning and their relationship to each other. This stuff is tight and has to be respected; positioning runs a range of about 3.5% of total bullet length and the correlation to each other about 1.75% of overall bullet length. Even the weight and arm of a gas check is considered. Hollow base slugs are grossly outside these limits.

Thanks for your comments Longbow, our good Lord lent his response to my dilemma as to go or stay. At 5:00 this morning, the power went out and was out for two hours. Not too big a deal, but it could have been. It was 26°, not a big deal for most, but a rarity in South Texas especially for this time of year. Our houses aren't built to deal with it. I have a backup generator, but there is a bit to getting the three homes off line and dedicating one to service. The setup is also more for a hurricanes and the hot weather around those events. I cannot run my central heat on it. I'm not sure he could do everything out in the cold on his own. The reality, Dad always trumps a nilgai. I'm getting ready to grill some pancakes for us, he did not even know the electric was out.

[Ranch Dog]

Hey Tom! Thanks for the explanations!

[Mr_Sheesh]

In model rocketry the usual way to determine CP is to cut a silhouette of the rocket out of cardboard and balance it front/back wise, the CP is where that silhouette balances, on the centerline of the rocket. As https://spaceflightsystems.grc.nasa....ket/rktcp.html

And to determine CG you just balance the actual rocket using a string (with everything installed so engine loaded, parachute/streamer in place, etc.)

Agreed that ballistics at trans sonic speeds will change external pressure and drag but I'm not an aerodynamicist. Once the bullet falls below the speed of sound, we're back to the CG and CP needing to be in the right relationship though...

[Ranch Dog]

I need to add that Mr. Tom Myers has taught me more about bullets and ballistics, in conversation and through his software, than any other source. Might be best said this way; thank goodness Lee Precision had cheap custom molds before I met him, some missed the mark because I wasn't paying attention to the details that he has taught me.

[megasupermagnum]

The problem with lightfield slugs, and other tail wad slugs is the terrible BC. They shoot about as flat as a round ball, maybe worse. I used to like the LBC sabot slug from BPI, it was a slug of a similar design to lightfield. I could launch them at 1950 fps, and they shot pretty good. The problem is that tail wad caused them to drop about 30"-36" at 200 yards with a 100 yard zero.

[Blood Trail]

In regards to 1oz Lee slugs, i believe why pose a fit to reload accurately is because it's a balanced projectile with the key design. I've been wanting to mod one of my lee molds to completely remove the key drive and make it a true hollow base. That way, it will make more nose heavy for stability.

[popper]

I know, this is posted about SG slugs. Velocity is a vector quantity. Break into vertical and horiz and it is almost never along the bullet centerline. Rising and falling bullets have the center of pressure changing due to 'slight' variations of vertical fps. Farther distance between CG & CP creates more torque to swap ends. That is why we spin bullets, gyro momentum is (usually) much greater than CG/CP torque. Rockets ( & VLDs) have a high BC/SD and don't need as much spin. The 'transonic' region differs between bullets and rockets. It really is the region where rotational momentum and forward momentum meet and allow CG/CP to take effect. Basically stating that rocket/plane dynamics and bullets differ. I won't even try tackling the SS shock wave variations.
Michael, hope the hunt and old folks stuff works out for your family.