Maximum weights

[Harter66]

Ive read the threads about revs stability etc and have a functional knowledge of spin vs stability. Also that shape is a variable for example wadcutters beginning to wobble at some point beyond normal target ranges.

Is there a weight point at which a given twist will fail?
To paint the picture a little more fully. Suppose we use the 1-66 of a 50 cal RB gun. Now nobody cares if a RB tumbles so let's believe that it is perfect at 2k fps with the nominal. 500 Dia 185 gr. What if the ball were a poly/tungsten ball and tipped in at 230 gr, would it still be stable enough to fly straight assuming 2kfps were still reached? In a secondary form what if the lead ball were given a flat base and 1.5 inch radius nose but retained the .5 length?

[leftiye]

Yes, in all cases if you go heavy enough your boolit will lose stability or tumble. Bad news is it seems to not always be the same weight for the given twist. Some bullets shoot fine in twists that would seem to be totally nutso. Witness the Marlin .444s with 1 in 38 twists. And even then, they all don't shoot well.

[Tatume]

It's not weight that determines stability anyway, it's length. A barrel that will stabilize a 220 grain 30 caliber round nosed bullet might not stabilize a 220 grain spitzer.

[cbrick]

It's not weight that determines stability anyway, it's length. A barrel that will stabilize a 220 grain 30 caliber round nosed bullet might not stabilize a 220 grain spitzer.

Yep!

It's the length of the bullet, not the weight that determines twist rate for stability. Weight only comes into the picture because in a given bore diameter it can only be made heavier by making it longer not larger in diameter. In 30 caliber a 150 gr bullet that stabilizes vs. a 180 that doesn't isn't because it's heavier it because it's longer. Many people use weight to say that in my gun a 200 gr won't stabilize but a 180 does but what they are saying is that the 200 gr bullet is too long for my twist rate.

Rick

[btroj]

Length is the key.
my 270 won't stabilize a 130 Barnes x bullet but it does well with all other 150s. The 130 X bullet is as long as a Horn 150 so it makes sense.

[44man]

I always figured it is the drive band length instead of overall bullet length. Longer needs shot faster to reach spin but weight limits the loads and velocity. The other way is a faster twist. The RB twist is slower and they will not shoot a maxi ball, need 1 in 48" and it will handle RB to boolits.
Had a 30-30 Contender that shot the RCBS SIL boolit with deadly accuracy to 200 meters, long bore ride but short drive. I ran out of checks so shot some without and they all turned sideways at 50 yards. Just that small loss of drive was critical.
I never got the Marlin .44 to shoot, needed a RB. The .444 could not even reach velocity and there were so many complaints they changed it to 1 in 20" but did not change the .44. I complained and Marlin sent me a copy of Greenhill!!!! Anyone knows that doesn't work.
I think the length of a wad cutter just needs shot much faster to keep stability. Even though not heavy, the whole boolit is drive length. Have to wonder if cast hard and taken to 1400 fps, what would they do at long range? I have no accuracy problems to 500 meters (547 yards) with WFN boolits.
BFR revolvers are a joy to work with and have a wide range of boolits that work, they have faster twist rates then any other revolvers.

[cbrick]

Not correct 44man. More bearing surface is always an accuracy benefit but it doesn't take the place of proper twist for bullet length. More bearing surface starts and keeps the bullet properly aligned, properly centered inside the bore. Once free flight is achieved adequate spin keeps it stabilized and adequate spin is determined by the bullets length, not weight. In free flight the length of bearing surface has nothing to do with keeping it stabilized but does with how stable it starts free flight. It is true that a marginally spun bullet can be somewhat helped by increasing velocity but only to a limit and it's a poor substitute for proper spin for bullet length.

The Greenhill isn't a bible, it's a guide, a tool. Too many factors can effect the exact spin a given bullet in a given bore needs to properly stabilize and the Greenhill formula used as "guide" will get you in the ball park. It was never meant as an end all final word and like many things in ballistics, casting etc. shouldn't be used that way.

Rick

[Wayne Smith]

Well, the Greenhill formula was developed for artillery shells, somewhat different than what we are shooting. There are a couple of adjustments to it to try to bring it closer to what we do. I have no idea how successful they are!

[Harter66]

Ive worked with a 762x 39, 308,and several 06's that show the length and drive length samples in spades . Had a 308 that was 1-12 at 175 gr it just quit period. 147,150,160,168 jackets pointy round nose cast it didn't care 2" or less it even shoots the 301618 at 172 gr ,175 gr 3030 fn shot gun groups. Two 1-10 06's 180s and 200s in 1 no problem the other 180s are ok 200s have to be long drive length or they yaw and wander off the paper. There was an 06' with a 1-8.5 that would shoot about anything you could close the bolt on as long as you didn't push to hard ,really from 2690 to 2710 the groups would open from an inch to 3" . A 200 spire point would hit a speed limit also but it seemed to be more a powder speed thing than the actual bullet speed. The little x39 with th Russo twist it doesn't care ,it's best is 3x5 whether it's a 312-155 or a 123 jacketed or a 200 gr spire point papered launched well I went to 1800 fps but will only do 1500 and fit the mag.

I guess the root of the inquiry is how does a weight gain in a bullet at its nominal length effect it's stability outside of aerodynamics.

I've an RBH that shoots a 262 gr SWC well enough for me to hunt with it and a 265 RNFP even better. But a carbine throws the SWC all over the place unless it is backed down just to slightly under Colts maximums my guess is that the RNFP will be even worse due to its greater length but it maybe just a bbl hum thing too.

The original question is does projectile density make a difference in stability?
Btroj you mentioned that 130 gr of copper doesn't shoot as well as a 150 lead or lead core. Is there an answer for why? That 270 doesn't like it? What if the 150s that shoot well had a heavier core and we're say 180 gr but still the same dimensions, balance center and shape as the 150s ?

[btroj]

The 150 Barnes is a far longer bullet. It wasn't stable at 100 yards, targets showed obvious tipping of the bullet.
Dont confuse accuracy with stability. Not all inaccurate bullets are unstable but pretty much all unstable bullets are inaccurate.
Stability is based on a relationship between bullet length and twist rate, to a large degree. Higher velocity can help overcome slow twist rate for a given bullet but not always.

Bullet weight doesn't really matter, it is all about shape. In 30 cal look at the length difference between a 150 RN, SP, and BTSP. All weight the same but the longer BTSP will need more twist rate to remain stable than the RN due to bullet length.

[MarkP]

I saw the Greenhill derivation several years ago, and density is in the equation. The equation that is commonly known is truncated and simplified.

PRODAS software is used to design projectiles, very complex calculations in the background.

[MarkP]

Spin is required to overcome the moment beween the CG and the CP (center of pressure). The greater the moment the more spin that is required.

[cbrick]

I guess the root of the inquiry is how does a weight gain in a bullet at its nominal length effect it's stability outside of aerodynamics.

That brings up the question . . . How could you get a weight gain in a given bore diameter without making it longer? OR . . . How could you get a weight reduction in a given bore diameter without making it shorter?

I guess I don't understand what point your trying to make because it's not bullet weight for a given twist rate it's bullet length for a given twist rate. Bullet weight has much to do with momentum, not stability. A heavier bullet may start out slower but it will retain a higher percentage of it's original velocity at a given range than will a lighter bullet. Then of course this could be taken to ballistic coefficient but that isn't the question, bullet weight is and you did say outside of aerodynamics.

Rick

[44man]

I never figured how to look at it, just guessing on my part. I always think of that 30-30 boolit, I did not change the length by more then the thickness of a GC.

[btroj]

I never figured how to look at it, just guessing on my part. I always think of that 30-30 boolit, I did not change the length by more then the thickness of a GC.

You changed far more than the length. Leaving the check off changes the bullets ability to take pressure and velocity quite a bit. If nothing else this example does show how effective a gas check really is.

[fouronesix]

Spin is required to overcome the moment beween the CG and the CP (center of pressure). The greater the moment the more spin that is required.

That's it exactly. And while sounding simple, it's extremely complex. The rub comes with predicting results. That's why even the "rocket scientists" test the computer generated models.

[Tatume]

That brings up the question . . . How could you get a weight gain in a given bore diameter without making it longer? OR . . . How could you get a weight reduction in a given bore diameter without making it shorter?

Change the density. High Power shooters used tungsten cored bullets for awhile, for a substantial increase in density and mass, and correspondingly increased ballistic coefficient. You can reduce density by using a thicker jacket, or by leaving a void in the core (concentrically located of course, such as a hollow point bullet).

[Harter66]

In my 30 cal examples the 180 RN was actually shorter than the 150s both in jackets.

Maybe I've tried to hard to simplify the question,making it more complex.
3 50 cal RB . 1 of copper 150gr (Unk just a guestomate),2 lead 185 , 3 a tungsten poly matrix at 230 . Loaded to a given velocity, obviously the heaviest ball will dictate the speed limit. The heavy will also carry more momentum for the given velocity. Which of the 3 is most likely to wander off the paper ? Or maybe what I mean to ask is will 1 be more susceptible to spin drift like a curve bal?

I chose the 50 cal RB as a basis because I takes so many other factors out of the mix by having a real world likely hood of it actually being testable. Maybe zink,copper and lead would make a better comparison with an adjustable mould to reach the stability limit of the lead then mill a bullet to match from copper and pour a zoolit. That would allow testing of the question. I don't have an iron mould but I do have copper rod,soft lead and zinc .

[texassako]

A good visual of this was when I helped someone out with some .257 Roberts loads in a Browning bolt action. 115gr Barnes Triple Shock keyholed, 117-120gr SP showed signs of tipping, 117gr round nose that I gave him to try shot into less than 1" at 100 yds. Practically the same weights, but the length difference really showed up between stubby and fat round nose, spire, longer spbt, and the really long barnes that even says to use 1:9 or greater twists.

[country gent]

The required Twist rate is a combination of several things all working together to get the desired results. Velocity, bullet length and also dia of projectile. The 1-66 twist for round balls in 50 cal are also the shortest possible usable projectile. There are several formulas to determine a good starting point but better is to ask what is working for others. As mentioned above some tungsten cored bullets were available for Awhile that were heavy for length. A 80 grn was basically the length of a normal 69 grn bt hp and would also stabilze in the 1-9 twist. Some of the 22 cal 90 grn vlds are requiring a 1-6.5 twist rate due to the length of the bullet and lowered velocities. So to add to question if diffrent materials are used your wieght range with the same length of bullet all should stabilize as long as velocity can remain at the same point.

[btroj]

I don't think changing weight for a given bullet shape and size is going to make a difference in stability at reasonable ranges. Going subsonic will but that is an issue for any bullet.

I don't think a round ball of lead is any more accurate at reasonable ranges than one of zinc. The zinc one will be affected by wind more as it will lose velocity far faster. This isn't a matter of stability but rather a matter of ballistic realities.

[paul h]

That brings up the question . . . How could you get a weight gain in a given bore diameter without making it longer? OR . . . How could you get a weight reduction in a given bore diameter without making it shorter?

I guess I don't understand what point your trying to make because it's not bullet weight for a given twist rate it's bullet length for a given twist rate. Bullet weight has much to do with momentum, not stability. A heavier bullet may start out slower but it will retain a higher percentage of it's original velocity at a given range than will a lighter bullet. Then of course this could be taken to ballistic coefficient but that isn't the question, bullet weight is and you did say outside of aerodynamics.

Rick

Bullet shape affects it's length. A wadcutter will be the shortest bullet for a given weight, a boat tailed spire point will be the longest bullet for a given weight. You can also go with a less or more dense material, a copper bullet will be longer than a lead bullet, of the same weight and shape, a tungsten bullet will be shorter than a lead bullet of the same weight and shape.

Now since this discussion is likely limited to cast lead bullets we can ignore the density of the material. If you want to design the heaviest bullet for a given twist rate, your best bullet shape will be an ogival wadcutter.

http://i62.photobucket.com/albums/h98/cascadedad/Gun%20Pictures/Bullets200711043.jpg

Which brings up another question, what is the maximum meplat dia that will give you an aerodynamically stable bullet at long range? IMHO somewhere in the 70-75% range is a safe bet for meplat dia. The larger meplats will be slightly shorter.

Final thought, it's better to pick a bullet design that is properly stabilized than trying to push things to the hair edge.

[btroj]

Actually Paul, shape has nothing to do with it. If by shape you mean identical size in all aspects then yes, different materials do make for different bullet weights. Yes, this is a technicality but in this instance it does matter.

As for meplat size, I don't know that there is a set answer. Velocity makes a big difference here from what I understand and read. Larger meplat seems to need more velocity to remain stable at longer ranges, for a given twist. This is an area where 44man can answer better, I'm sure he has tried a wide range of meplat sizes and velocities.

[paul h]

Shape affects the length of a bullet for a given weight, so shape definitely does have something to do with it. When you are getting at the limit of what bullet length a given twist rate and velocity will stabilize, going from a spire point bullet to a wadcutter (extreme example) can get you from an unstable bullet to a stable bullet.

The reason I mention meplat dia, is that for a given weight ogival wadcutter, the bullet with a smaller meplat will be longer than the bullet with the larger meplat.

[leftiye]

Like I said (and I did/do know that length is the issue, he asked about weight), if you go long/heavy enough your bullet will destabilize. But many cast boolits seem to stabilize when it should not work, while many that should werk fine don't. These previous examples illustrate that it isn't as simple as greenhill's formula. Maybe if it shoots well in Mr. Gun, you can't argue with it?

[44man]

Whatever the answer is, the relationship of twist and velocity will not go away. A long or heavy bullet just needs the correct velocity or twist and that makes a slow twist and the restriction on how fast you can go as loads must be reduced for weight.
Some of my revolver boolits have 80 to 82% meplats. My best and longest boolit is the 330 gr, .44 WLN but it still has a 79% meplat. That is the boolit that did 1-5/16" at 200 yards. Drop is another thing with it dropping 35" out there but it never goes unstable. It needs a little over 1300 fps and can't be shot slow. My 420 gr, .475 boolit has a short nose, 80% and fits a Freedom but needs driven to max pressures but the faster 1 in 15" twist of the BFR does not need much over 1300 fps. The 440 gr in the JRH, also a short WFN shoots best at 1350 fps, again 1 in 15".
Round balls are no problem. I shoot my .54 with 100 gr of Pyrodex and good to 100 yards but for 200 I raise to 120 gr and can hit steel chickens at 200 meters. Just increase spin.
In my .50 I could shoot one ragged hole at 50 and killed deer to 125 yards.
Any formula I ever tried measured boolit length and every single answer showed a twist way too slow so guys changed the formula to match what they use but it falls short for other guns.
Greenhill was made for BP cannons, most likely round cannon balls or the first rifled cannons. A far cry from computations made for modern cannons or other guns. Amazing they get sabot rounds out of a smoothbore but there are fins to spin.
You can't make Greenhill work unless you change input. Garbage in garbage out.
The only way to see what works is to shoot.
Have you seen the new rifle sight? The sight and laser controls the trigger and the gun will not fire unless the dot is in the correct place on the target. It does not miss.

[44man]

More velocity means more spin, no matter the twist so a 180 gr might need shot faster then a 150 gr but you can't do that. The answer is a faster twist. The lighter bullet can be shot slower. The heavy can be shot within pressure limits.

[Toymaker]

What??? You've never had a round ball key hole????

I love that line. No joke, I once had a Chief Range Officer stop a muzzle loader competition to haul our butts down range and show us where our round balls were key holing and tearing up his target frames.

[44man]

What??? You've never had a round ball key hole????

I love that line. No joke, I once had a Chief Range Officer stop a muzzle loader competition to haul our butts down range and show us where our round balls were key holing and tearing up his target frames.
That is funny! :bigsmyl2: I guess I have had it happen, a key hole from 5 shots at 50 yards.

[BAGTIC]

One could use a denser material or a shorter ogive or a wider meplat.

[BAGTIC]

Might work in extremely marginal situations. The problem is that the force acting to yaw the bullet is air pressure on the nose and as velocity increases that air pressure increases so it comes out pretty much a wash.

[BAGTIC]

Some bullets can be deformed by firing, soft lead ones quite easily. The usual result is fore shortening cause by bullet setback and the expansion during obturation. It is the after firing dimensions that matter not the before firing.