megabutter

I came across this when surfing around and what not. Thought I'd see what anyone knows about it.

"If you over pressure any round object it will become unstable. Just the right amount of gas is needed to get the a more accurate shot, sometime less is more."

Anyone have experience with this? Is there a known formula for barrel length, shot weight, and muzzle velocity that deals with this?

Thanks!

arcticap

I think that rather than pressure, what the statement is referring to is velocity. Pressure and velocity have some overlap, but different powders have different pressure curves which aren't necessarily related to the velocity of the projectile in the end.
Now what makes velocity important is how that affects the revolutions per minute (RPM's) of the projectile which in this case is a round ball.
That means that the greater the velocity of the projectile when it leaves the bore, the more that it will spin.
Every projectile needs to spin at a different rate to stabilize and to fly straight. Long, heavy bullets need to spin faster with more RPM's and velocity to be stabilized.
Short projectiles like round balls and light bullets require less spin to be stabilized in flight, therefore they fly straighter with less powder, velocity and RPM's.
But what throws a real monkey wrench into the works is the twist rate of the barrel. The faster twist barrel may impart too much spin to a round ball, especially if too much powder is used which results in too much velocity and RPM's.
That's why fast twist barrels are better for stabilizing heavier bullets. But by using less powder and slowing down the RPM's and velocity of the ball exiting the barrel, a fast twist can satisfactorily stabilize a round ball. Just don't use too much powder.
However with a slow twist barrel, more powder must be used to stabilize the ball by creating enough velocity and RPM's.

So the lesson of the statement is to not use too much powder or else the ball may be destabilized and spun too fast.
That's why heavy hunting loads with round balls may not be as accurate as light target loads.
Hunting loads are sometimes a compromise between accuracy and power. While target loads can be more accurate but lack some of the power needed for killing an animal.

This is all why it's important to incrementally work up a load starting with a lower powder charge and slowly increasing it in 5-10 grain increments until the accuracy is satisfactory and the groups are tight, which indicates that the right amount of velocity and spin is being imparted to the ball.

Of course the same method of working up a load also applies to testing bullets.
But with the benefit of experience, a shooter can estimate how much powder to begin testing loads with based on the twist rate of the barrel and the size, length and weight of the projectile.

More velocity equals more RPM's.
Less velocity equals less RPM's.

Balls require less RPM's to stabilze
Bullets require more RPM's to stabilize.

Slow twist means to use more powder.
Fast twist with a bullet means to use more powder
Fast twist with a ball means to use less powder.

This is due to how many RPM's is required to stabilize each shape of projectile based on it's length.
This is based on something called the Greenhill formula which helps to determine the best barrel twist rate for a projectile of a given length.
(Scroll down the page to twist rate - http://en.wikipedia.org/wiki/Rifling
Every size (caliber/diameter) ball has a favored range of barrel twists. But the favored range of twist rates can sometimes be fudged by controlling the velocity and RPM's.
Barrel length only affects velocity by a small amount of feet per second for every inch of additional length over a standard length barrel. So the amount that the barrel length will affect velocity and accuracy isn't usually too great, but none the less it's usually something that needs to be determine by range testing.
Welcome and good luck megabutter with applying any of these basic ballistic principles.

nchawkeye

With my deep cut round ball barrels velocity is not a problem...I've used from 50-120grs of FFF Goex in my .54 without the groups opening up...Once you get to the higher powder charges you might have to put hornet's nest over the powder to keep from burning through the patch...

With real black powder and the proper ball/patch combo in a round ball barrel you aren't going to push the ball too fast...

UncleNorby

A barrel can only impart the twist it has to the bullet (twists per inch or travel). Increasing the powder charge will generally increase the velocity, but will not change the number of rotations the bullet makes as it travels over a given distance. That is "programmed" by the barrel. If you shoot a 50 grain load your bullet will rotate within the barrel a given number of times. Does that change with a 100 grain load? No, the only thing that changed is the time it took to travel the length of the barrel. The flight time to the target will also be reduced (though a very small amount within the range of hunting loads), so the bullet will make the same number of rotations in minimally less time. If you want to look at that as more RPM, ok, but it is not more rotations between the muzzle and the target.

As stated above, longer projectiles require more twist than shorter ones of equal diameter.

I'm no physicist, but I can't think of any reason a perfectly round ball would be more or less stable with a change in twist rate, if all other things are equal. But, that's where the problem is, in all the other things that are not equal, like the ability of the patch to impart spin as pressures increase, barrel harmonics, the actual concentricity of the bullet and all the rest of the imperfect things in the shooting system.

So, with a given bullet and a given gun, there will be a charge that stabilizes the bullet optimally to overcome a high percentage of the imperfections in the system. Bullet stability may decrease above or improve below that optimal charge. A gun may also have more than one sweet spot for a given projectile.

Semisane

The missing factor in the discussion so far is the ability of the projectile to grip the rifling at various velocities.

A cartridge rifle can have a twist as fast as 1:8 because a hard jacketed bullet that is slightly larger than bore size is swagged into the rifling and the bullet is so tight that it will grip the rifling and spin. Try to push a lead bullet too fast down a fast twist cartridge rifle bore and it will "strip the bore" (i.e., lead will be shaved from the bullet, filling the rifling with lead) and essentially shoot like a smoothbore.

In a muzzle loader the projectile must be the same size or smaller than bore size, so it's impossible to achieve the tight fit of a cartridge rifle. A patched ball shot at high velocity from a fast twist looses accuracy because the patch isn't be able to stand the pressure and is torn asunder.

arcticap

I disagree because I believe that the RPM's are increased after the bullet exits the barrel when loading with more powder and velocity instead of less.
That's because the bullet will spin faster due to the increase of inertia/momentum from being launched through the barrel at a higher rate of speed.
IMO the bore acts like a centrifuge on the bullet rotation. When the bullet zips through the barrel faster, it will rotate faster after it leaves than if it was propelled slower and by less powder.
The bullet is no longer constrained by the original twist rate inside of the barrel but is free to rotate faster as dictated by the effect of the increased momentum and velocity.
I believe that's the reason why some some powder loads and velocities will stabilize a bullet while other loads won't. Everything is basically the same except the amount of spin imparted to the projectile which affects bullet stability.
It's mostly based on the velocity and related RPM's needed to stabilize a specific bullet profile exiting a barrel having a specific twist rate. Different twist rates besides the optimum can often work if the powder charge and velocity is adjusted.

Semisane

I'm not a physicist but I once slept in a Holiday Inn, and still don't know which is correct. I'd sure like to know. Can either of you prove your view with an authoritative reference?

lemoyne

Semi
Under Stabilizing a bullet and it will tumble this cause instant inaccuracy it is much harder to over stabilize a bullet, it can be done but usually if you have the right twist for the caliber and type of bullet you won't.
If you blow the patch or sabot or strip the bullet it will shoot wild.
Most of the time what you are dealing with on high end loads with a muzzle loader is the whip or vibration of the barrel. A rifled barrel whips with a circular motion and the most accurate point is when it comes back to the point it started from because the barrel does not impart any side ways momentum.
They had a high speed picture that they showed in slow motion when I was an apprentice that showed the barrel whipping in a circular motion and they explained that that motion is added because of the stress of spinning the bullet.
I know a lot of you will find this had to believe but there are usually two or three loads that will shoot accurate and they are departing the barrel at the original position, the heaviest load will be the hardest to find because the barrel is whipping faster with a heavy load, heavy barrels slow this down and whip less which is why it is easier to get them to shoot accurate.

sabotloader

I found this article awhile back when I was searching for something.... It might fit right in in this discussion.... Interesting what he says about 'over stabilization'

I have always wondered about 'over stabilization' and the wobble thought really does seem to be what I see... Is it possible for mass produced bullets to be perfectly round and balanced?

bronko22000

Arctic - you are correct that increased velocity will indeed increase RPMs. But Semi is also correct in that if a bullet is fired out of a 1:48 twist it will rotate one complete turn in 48 inches regardless of velocity. Now, it you increase velocity the twist will remain the same (1:48) but it will be traveling faster downrange therefore covering the 48" increments faster - hence spinning faster - resulting in more RPMs.