muzzle velocity vs accuracy
 

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!

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. :guiness:

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...

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.

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.

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.

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?

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.

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?

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.

I think I pretty much understand the twist/bullet lenght/stabilization thing. But I'm still interested in resolving the different view points expressed by UncleNorby and Articap.

If a bullet is shot from a barrel that has a 1:48 twist, is it possible for it to increase its rate of rotation after it leaves the barrel? That just doesn't seem possible. It seems it would rotate once for every 48 inches of travel. The issue is number of rotations per unit of distance vs. number of rotation per unit of time. Actually, rotation must slow down over both time and distance due to air friction.

Clearly, a faster bullet will have a higher RPM (time). But how can shooting a bullet faster cause the rotation to exceed the twist of the rifling (i.e. one rotation in 48 inches)?

(EDIT: Bronko got his answer in before this hit the screen. I agree.)

bronko22000

OK - I am having trouble with this one.... Would not the Revolutions Per Minute be consistent for only a short distance out of the barrel and then start to decay with distance. As it decays it reduces RPM. The trick is to get it to the distance you want it get to before velocity and RPM's have decayed to the point that accuracy is lost...

Granted as Bronko says with greater velocity it will cover the distance quicker before decay become as factor.

But I am still thinking velocity and RPM's will reduce over a given range????

Thinking that is where I always get in trouble..

Yep. But velocity decays faster than RPMs.

SL, velocity degrades much faster than RPMs.

all that is correct but a lot of it does not apply. Most muzzle loaders do not push the bullet 2000 fps a few heavy loaders and smokeless shooters that use a muzzleloader will push up to 2400 fps at these velocities a 1- 28 or a 1-24 depending on the length of bullet will do the job what amazes me is that people using 70 or 80 gr and shooting 1350 to 1450 fps get such good accuracy, one thing in there favor is that there is very little stress or barrel whip with the very light loads.

Yes, i agree with both of you but the fact is RPM's will decay. So in my thinking the greater the velocity the longer the projectile will retain the RPM's to remain stable from a given twist??????

lemoyne

But, Lee I think that goes right back to your shooting background... when you were shooting bench competition were you shooting for velocity or accuracy? I believe greater accuracy (consistently) can be had with a reduced load from what you might use hunting...

I'll buy that (and I want a side order of fries), though I suspect it's not an issue except with bullets that are "longish" for their caliber.

From a given twist, the higher velocity will give greater RPMs and therefore deterioration would obviously take longer..

Unlike some of our threads. :s2::s2::s2:

Big Z

I do not think velocity has anything to do with RPM's that is set in the bore and decays as the bullet travels.... however, velocity will the get the bullet a greater distance before the RPM's decay to instability

I think!

Wake up Sabotloader. You're lagging behind the class. :biggrin: Revolutions per distance is set in the bore. Reveloutions per minute is a function of both twist and velocity.

Let's slow everything down. If you shoot a bullet out of a 1:36 twist bore at a velocity of three feet per second, the bullet would travel three feet in a second and revolve once during that second (i.e. 60 RPMs). Now shoot that bullet out of the same bore at 6 feet per second the bullet would travel six feet in a second and revolve twice during that second (120 RPMs). It's still rotating one time for every thirty-six inches traveled, but it's traveling twice as far in one second, so higher velocity means higher RPMs - but still one revolution per 36 inches traveled.

Semi

I thinking....

I did think... i really tried

I'll read this again tomorrow... I am baby sitting my 2 1/2 year old grand daughter all by myself - I am not thinking well when it comes to high math...

OK Grandpa. We don't want to distract you from the important stuff.

My youngest granddaughter is seven and her sister is twelve. Delightful ages also, but I wish I could start them over at two again. The twelve year old starred in Samantha's First Turkey (LINK: http://huntingnet.com/forum/black-po...st-turkey.html) What fun they are.

Semi

OK it is new day, the diaper is changed she is dressed and playing in the living room, later we are headed to 'Frog Pond' play area in the mall...

but! I think i have it now... RPM does increase with velocity over a given distance and ROT 'rate of turn in a distance' is set by the twist of the bore - am I getting warm???

Semi

The way I look at it, the only thing making a bullet spin is the influence of the rifling. Once outside the bore, that influence is eliminated and spin can only diminish. Once started spinning, the bullet does not "want" to spin faster or even keep spinning at all. The RPM and rotation per inch decrease as the bullet travels downrange.

And that's why a bullet can be accurate as heck out to a certain distance and then loose accuracy beyond that distance. The closer a bullet is to the necessary rate of twist for stabilization when it's launched, the shorter the distance at which it looses stability and accuracy.

Mike you really can only separate them to a certain extent. I used 160gr of cartridge grade pyrodex in the gun I won every thing with, it had a 40 inch barrel and the velocity was part of what gave me an edge.
I give you the example of my 2400fps load with the Lehigh 200gr 40 caliber bullet shooting 1/2 to 3/4 inch groups when I am shooting good. The length of the bullet and the twist of the barrel and the whip of the barrel control accuracy as long as the velocity allows all to match up. Take the cartridge guns that shoot 4400 FPS and put all the bullets in one hole as an example if speed caused inaccuracy that could not work.
Also the speed of rotation has been checked with marked bullets and high speed cameras the speed of rotation slows very little after the bullet leaves the barrel, and as for it speeding up after it leaves the barrel thats a fairy tale.

What is the RPm of the younger granddaughter when she is on her sit & spin?

Does an object in motion tend to stay in motion unless acted on by another force? Just askin'!

I can tell you one thing Old/New, if you give the younger one a ten minute nap and a brownie when she wakes you can double her velocity. .:happy0157:

Me thinks we're getting too involved in this. Maybe we just continue to make smoke and have fun...

That's funny...plus I got my 2010 Census in the mail today. Now I'm really laughing. :cry::happy0157: