The Relationship of Center of Gravity and Pressure with Slugs?

[W.R.Buchanan]

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.

Thanks for the explanation. Still don't fully comprehend the CP. Is that a computer generated value?

The relationship between the CG and CP? What is optimal? Or is there an optimal? And does this change with the type of bullet profile? IE: long and skinny vs. short and fat.

The actual design of the Lyman Slug closely follows the average Diabolo Airgun Pellet and that design has been proven to be very accurate in Airguns within a certain range (10M). I have several that will essentially put all their shots thru the same hole for as long as you can stand to do it. The greatest accuracy seems to be delivered at a velocity of around 600 fps.

I wonder if there is a velocity "Sweet Spot" for these larger pellets as well? And that maybe we are pushing that envelope and trading Accuracy for Horsepower?

From a smooth bore It seems that the Lyman slug will only produce whatever accuracy is granted by divine guidance. However from a rifled barrel I would expect it to act like an Airgun Pellet. Lets face it the slug has the Aerodynamics of a Refrigerator, but that doesn't mean that it can't provide a useful service within the ranges it is designed to be used at. It is just NOT going to be one hole accuracy. Too many variables to unravel. So you take what you can get.

This all comes under the heading of the statement I have made here and elsewhere so many times regarding Accuracy of different weapons.
Mini14's come to mind as I have fought that battle many times. There is "wished for accuracy," and there is Reality. Only one is useful.

Many people regard the Mini 14's accuracy as completely unusable. And in fact think that every gun must exhibit sub MOA accuracy in order to be useful.

We all know that this is a stupid way to look at it. My Mini 14 is a solid 1.5" gun and It will kill you just as dead as a 3/4" gun or for that matter a 3" gun. By extension these shotguns are only going to be 100 to maybe 125 yard guns and if we can get one inside 3" at 100 yards I would say we had reached true nirvana. But twice that would probably do the job as the size of the projectile makes up for it's inability to hit in the same place every time.

It all gets down to the end result that you work up a load and then you go shoot something with it.

If you end up eating what you shot, then I would say you were successful.

Randy

[Ranch Dog]

Thanks for the technical stuff popper and thanks for your kind words. After the power was up, I made my dada good ranch breakfast!



I told him I'd rather hit the road with him, a couple of day trips around South Texas than go on the hunt, so that is what we are getting ready to do. We are going to visit some country we haven't seen in a while, just out and back as he likes being home at night.

[Ranch Dog]

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.

As long as they are predictable, I don't have a problem with the drop. Not much difference that a 45-70 Govt with a heavy bullet, you got to have a dependable MOA though.

Just finished the shooting for the 680-500-HB. Great at 50-yards, better than then Lee. Still, MOY (Minute of Yardstick) at 100-yards.

For grins, I had pulled the LightField components and shot the total projectile with the same load (25.6-grains of International, image says 25). I shot them immediately after the 680-500-HB with the same POA. Two in the same hole.









When that was done, tried the 2 3/4" Lightfield from the used cylinder bore, rifle sighted barrel I just bought. I didn't clean the barrel, didn't fiddle with the sights, didn't do anything. Just put the barrel on and shot it at 50-yards. All three shots touched.



The only thing I don't like about the Lightfields is that those three hulls represent seven dollars and fifty cents. Of course, that is a drop in the bucket compared what I've spent trying to get a home cast slug out to 100-yards with a huntable MOA.

I got to go, but when I come back, I'm going to drop these old designs and do something different.

[popper]

Yea, have fun and keep them happy. That breakfast would put a smile on my face. Mom's 99 and not in very good shape, in a mental care now, really sad to go visit. Enjoy while you can.

[megasupermagnum]

I hope I still look that good at 90.

Sure the drop is consistent, gravity doesn't change. I just plugged my personal results into the Hornady ballistic calculator. It looks like a BC of about .06. With just a 10mph wind, calculated drift at 200 yards is 65"! Maybe Lightfield slugs are a little better than the LBC, but that tail wad is a killer. No matter how you cut it, I just can't come up with a slug that's good for more than 125 yards. I've recently been turned on to a factory load, the Federal "sabot slug". It is a swaged 1 ounce hollow point. So far results are fantastic, sub 2 MOA at 100 yards. They sell for $7 a box too. I'm going to be trying these at longer range this winter.

[W.R.Buchanan]

After seeing what a Lightfield Slug actually looks like I can see why they shoot so good. They are drag stabilized, which is something we haven't talked about much.

It is also a Wasp Waisted design, but the kicker is the attached wad which uses the hose barb type of attachment like the various Russian slugs we saw a few months ago.

I personally think the attached wad is the way to go for accuracy. And I can see it working for both smooth bore and rifled barrels.

On another more recent note ; I'd like to see how the CG/CP relationship is on those slugs and compare it to our Lyman Clones.

Randy

.

[popper]

CP is a drag induced number. Frictional drag, depends on fps, is virtually equal on all Diam's of the slug so has little influence on stability, just slows it down. Lite field slugs have the tail in airflow that puts the CP way behind the CG and adds stability. Shuttlecock theory.

[megasupermagnum]

I just looked at the pictures. Lightfield uses Bluedot powder. Whodathunck it. What was the charge weight of the factory Lightfield load?

[longbow]

Ranch Dog:

If you are thinking of something new and different for slugs you will likely be better served by a full bore slug.

I'm no wizard here and am still struggling with slugs in smoothbores so take this with a grain of salt...

Greg Sappington was working on wad slugs quite a number of years ago and he asked me to make him a mould to cast a slug he designed so it would fit a steel shot wad as a discarding sabot. It worked but he made some changes so i made him another mould. Same basic design and still to fit into a steel shot wad (maybe CSD wad?). While both slugs worked and terminal performance was what he wanted, accuracy was not as good as he wanted and that mostly, if not all, due to the inconsistencies in the shotcups. They just aren't made to sabot standards and tolerances. In the end he rethought things and went to a machined Delrin sabot and sub bore slug.

Alternately getting rid of the shotcup or sabot and going to full bore slug will accomplish the same thing by removing the inaccurate component... the shot cup.

Since I like to do things myself with what is readily available and little if any specialized equipment of components, I would go full bore slug. Accurate moulds has quite a collection of full bore designs now so good selection. However, the slugs I like are the old Dixie Tusker and the Paradox bullets.

The Dixie Tusker was a very nice slug in my opinion. It was 600 grs. and hollow base though it was intended for rifled gun. James Gates posted the drawing here for others to use which was very generous of him. I think James used the hollow base to provide more bearing length without raising weight too much. A very nice design to my eyes.

The Paradox bullets always fascinated me so I'd get a mould made just because I like them. They are a bit heavy at about 740 grs. so load data is a bit sparse but I like them.

Another option is the Hammerhead slugs. You have to buy the special wads but you can buy a mould and cast your own slugs. I like that. Also, Blood trail says they are very accurate so that is a good endorsement for those slugs.

Just some food for thought for a guy who wants to try something different.

Longbow

[OnHoPr]

Good evening gentlemen. I think you are kinda putting the cart before the horse on the projectile technology thesis. Especially with wad style slugs. The better ones are the Buckhammer style slugs because their sabot style sides have greater thickness in the material used and can handle the jump and travel from hull to end of barrel. Though, I still believe that the wad cushion prevents them from becoming more accurate. I think the wad base will always have burdens too. Should it drop or should it stay attached. How concentric can you make it in alignment with the slug. How can it endure ignition and down the barrel. The seemingly easiest is the shot cup slug, but it seems to be the most difficult to keep consistent not only with the cushion, but the petals as well. The ones I seen that seem to have hopeful expectations were with steel wad cups with no cushion. But, trying to get those concentric on wall thickness created a problem as well as the diameter of the slug with overall diameter and shoving in a hull.

All of these slugs have like sheeeit for BC. You have to have less than 3 mph winds just to see if you can get in the 2 MOA range. Maybe they designed the slugs to large for the thin clays & field load shot cups and need something in the .630 to .650 range for the steel cups for durability. If you go full bore with the heavies you get some heavy duty recoil. I guess they make a few lighter full bores, but then you have to have your forcing cone, lead, throat, whatever designed well enough to not damage the slug from ignition. Something like the Hastings Laser slug sabot, like the Buckhammer and Lightfield, but just the sabot and cast the slug while working with your own wad column.

Edit addition, Slugsrus sabots.

I mean to suggest, have we come very far at all in the last 60 + years when it comes to shootin the slug venues. I am not talking about rifled barrels and saboted bullets. Take a 60s' Deerslayer, put a 4X Weaver on it with one of the three major manufacturer foster slugs on a calm winds day and it will shoot 4 MOA off the bench ALL DAY LONG.lol You would think adding rifling to a barrel the MOA would shrink wouldn't you. Heck, I just may be thinking a little to critically from the general 50-60 slug gun to the consistent 100 yd slug gun and the 75-80 slug gun is a step in between while the 125 yarder is something dreamed about. Something like shootin the 300 savage to 200 yds pretty easily to the next steps of 350 yds and then to 500 yds might have some difficulties.

Lot of criteria to think about though. From individual guns and their barrels to all the components and reloading practices.

Oh, and Ranch Dog, how are you suppose to shoot good when you have troubles keeping your pancakes round.

[Mr_Sheesh]

Putting a wad or other light object behind your slug will keep the CP at "midpoint" of the slug; The wad being light, the CG will indeed be WAY forwards of the CP. That's why attached light wads stabilize slugs well

If you put another lead slug behind the slug, it wouldn't stabilize the slug. (And would increase pressure if you didn't reduce the powder charge etc.) That'd move the CP back, but also the CG.

I have to admit that I've wondered how they calculate CP for a "grid fin" like the MOABs use; It would have a lot more effective rearward movement of the CP than the cardboard outline method would indicate. But I am not planning on making a firearm to launch a MOAB any time soon, I'll leave that to the USAF

[Cap'n Morgan]

It is hard - maybe even impossible - to design a full-bore 12 gauge pure lead slug with the Center of Gravity sufficiently forward to stabilize the slug from a smooth bore without having some sort of wad attached. I have tried all kinds of tricks (when designing in SolidWorks CG is calculated on the fly) You can add a hollow base, but only to a certain depth, then CG will move backwards again. You can make the skirt thinner, but this will weaken the base. You can go for a flat nose design (wadcutter), but this will create a poor aerodynamic flow and cause the slug to tumble at some point down range.

This curly headed airgun guy has a pretty good explanation on the difference pellet designs and why they need different twist to perform well. Of course this is not smooth bore guns but it's still quite interesting:

https://www.youtube.com/watch?v=Ur6iIcVhHmM

I'm still trying to find some time to test my latest HighSpeed & LightWeight SuperSonic (TM) slug design. Any day now...

[popper]

MOAB's grid fin is actually a bunch of 'vertical' stabilizers that are steered by angle off the perpendicular quadrant. Low friction so fps is not reduced. Doesn't change the C.P. merely the torque applied. Side fins prevent rotation so the guidance system works. Many ordnance use the grid fin as it only requires rotational movement - force on it is not increased as angle of attack type fins. LEGAP used fixed rear fins with canard 'swivel' head fins for steering. The 'curly' headed guy talked about over stabilization which is really the angle of attack increase (due to gyro effect) and vertical air movement creating torque around the CG - CP does move in this case. So for long range twist, you want the CP moving toward the CG to reduce torque. Rockets on the other hand, CG changes and should move toward the CP which doesn't change in 'powered' flight. All projectiles are 'drag' stabilized, WC have the force applied at the nose, VLD is an attempt to move the 'drag' back to the rear. Attached 'wads' create 'drag' at the rear.

[W.R.Buchanan]

This just keeps getting better everyday. I am actually learning stuff I didn't know. Keep going!

Randy

[longbow]

I am still a Brenneke Classic fan, largely because they work well and because, potentially anyway, I can make them at home. The main problem I have had is maintaining the integrity of the wad column/attached wad.

I have not tried dense felt yet but will be ordering supplies shortly so both felt wads and BPI plastic "gas seals":

https://www.ballisticproducts.com/HB...tinfo/072HB12/

will be included. I think I'd have to stack 2 of those to have enough attached wad.

Talking smoothbore here.

Now a question or two for the smart guys:

- just how much back of CG should CP be for good drag stabilization?
- can you add too much of a good thing? As in is there a maximum recommended attached wad length for stability? (and of course we're talking about something that will fit into a 2 3/4" hull, not an arrow). turbo posted recommended diameter to length proportions for sll lead HB slugs and attached wad slugs but I don't know the source or if entirely true ~ Hammerhead answer again.
- is an attached wad detrimental to rifled gun accuracy? Brenneke claims their Classic and other attached wad slugs are good in rifled guns too. Hmmmm, I guess if Hammerhead slugs are accurate from rifled guns then maybe I've answered my own question.

Mihec made a full bore version of the Lyman 525 gr. sabot slug but I haven't seen any range reports so no idea how accuracy is.

I do have to say that in my experience loading RB's and slugs into shotcups for smoothbore is easier than building wad columns and seems to give me better accuracy... with the exception of 0.735" RB which equals accuracy of RB in shotcup loads. I have had better results using slugs in shotcups than full bore slugs though. I think due to added guidance into the forcing cone/bore and also by protecting the slug from damage bouncing off the forcing cone.

Longbow

PS: Randy, I've been "learning about slugs for about 40 years and still apparently don't know much except it appears that the simple lead slug is smarter than I am!

[Cap'n Morgan]

Now a question or two for the smart guys:

- just how much back of CG should CP be for good drag stabilization?
- can you add too much of a good thing? As in is there a maximum recommended attached wad length for stability? (and of course we're talking about something that will fit into a 2 3/4" hull, not an arrow). turbo posted recommended diameter to length proportions for sll lead HB slugs and attached wad slugs but I don't know the source or if entirely true ~ Hammerhead answer again.
- is an attached wad detrimental to rifled gun accuracy? Brenneke claims their Classic and other attached wad slugs are good in rifled guns too. Hmmmm, I guess if Hammerhead slugs are accurate from rifled guns then maybe I've answered my own question.

I copy pasted this from the patent papers on the Vitt slug:

https://patents.google.com/patent/US3200751A/en

With reference to FIG. 8, these factors are the location of the center of gravity CG and the relative location of the center of pressure CP. Actual reduction to practice, shows that the optimum distance along the central axis between the nose of the slug and CG in proportion to the total length of the slug must be a minimum of 0.25 and a maximum of 0.35 of said total length. Cooperating with such favorable location of CG, the center of pressure CP must lie on the central axis far enough behind the CG to assure head-on stability of the slug during the short interval of time before the gyroscopic effect takes over. This separation between CG and GP for the 12 gauge slug, for example, must be from 0.3 to 0.4 inch, and the ratio between the dimension of this separation and the total length of the slug must be between a minimum of 0.15 and a maximum of 0.22 for any gauge or caliber of shotgun slugs.

[longbow]

Cap'n Morgan... you are the man! Again! Thanks for that. If I had a half a brain I guess I could have searched and found that but years of 12 ga. recoil have left me with somewhat less than half a brain.

I did search after reading that and found some other interesting info but that patent had the most info on CP and CG.

I believe I will re-initiate my intent to steal Wilhelm Brenneke's thunder... or George Vitt's thunder (very similar design to Brenneke). I've just sized down 30 1 oz. Lee slugs and made forms for hot melt glue skirts. This worked well on as cast Lee slugs before but they were hard to line up in the form due to taper and the glue was a bit messy at the tail end of the slug due to gap. if I was a better machinist I guess I would have made a tapered "socket" to suit the slug then the skirt would have matched up in diameter.

The sized slugs are straight sided so no problem to match up the glue skirt.

I will now do a bit of research and apply the CP/CG info to the design.

Longbow

[Ranch Dog]

Woow... great information. Been tied up a bit with family, but very interesting.

I've gone on to a full-bore/groove bullet and Sabot Technologies Thunderheads; mold and sabot columns for now.

[popper]

So basically a push-pin shaped plastic part that attaches to the cast slug via a shaft through the middle of the slug and is rivited at the front?