JRow

After studing guns for the past 6 month (my total gun ownership lifespan), a couple dumb questions come to mind:

1) What does the direction of the rifling twist have to do with the performance of a bullet?

2) What percent of the pressure is lost at the cylinder gap on a revolver? Is there still pressure in the barrel after the bullet has crossed that gap? And a follow-on question, does a longer barrel result in more or less muzzle velocity?

Thanks - JR

BronxBoy

There are no dumb questions... well, yes there are but these are not among them!

1) Someone with more experience in guns will have a more refined answer but from a mechanical point of view the direction of the twist should have little to no effect on the bullet performance. Spinning the bullet about the center of mass stabilizes it’s trajectory and produces less air resistance, generates lift (Bernoulli effect) and produces greater distance. Whether a twist is left handed or right handed should not matter but the rate of twist does make a difference. A right handed quarterback has no advantage over a left handed quarterback in the ability to spin the football but a football that is spinning too slow will have poor accuracy and distance and one that is pinning too fast will have poor distance since much of the force of throwing the ball would have been directed into its spin.

2a) The answer to this depends on the gap and a gunsmith is better able to give you quantitative data but laminar air flow principles dictate that the majority of the forces generated by the expanding gasses will be directed into the barrel. I too would love to hear the specifics of pressure loss due to gap size but it will probably be much less than you may think! Revolvers do not utilize any of the forces generated to cycle the gun as is done in a semi-automatic pistol. Power losses are somewhat minimized.
2b) Pressure is applied to the bullet for as long as the gas that drives it is still expanding. So, yes, there is still pressure on the bullet while it is in the barrel.
2c) While there is a limit on barrel length vs. velocity that I am not qualified to discuss, generally, the longer the pressure is applied to the bullet the greater the acceleration. Longer barrels produce greater muzzle velocities than their shorter cousins.

There are some very good gunsmiths on this forum that can add to this and speak specifically to firearms but the mechanics are fundamental.

Varmiter

Just to re-inforce BronxBoy's comments:

The direction of twist is irrelevant. Now the rate of twist is much more important. In handguns, twist rates are incorporated by the manufacturers for the range of bullet weights/lengths the shooter may encounter. About the only time a problem may occur is if the shooter is attempting to shoot a very light load, and the twist rate is unable to stabilize the bullet due to a reduced velocity. There are many graphs/tables for twist rate stabilization.

Generally.....repeat....Generally, max pressure is achieved before the tail of the bullet passes the gap. While the there is a loss at the gap, the bullet still continues to accelerate for the length of the barrel.

Finally, Yes, generally, the longer the barrel, the higher the velocity, within reason of course.

Chris

Iowegan

I agree with everything above except a few of BronxBoy's comments. Bullets don't have lift ... they have drag, which makes them drop at a very predictable 32 ft/sec/sec, the speed of gravity. The spinning effect of a bullet does not generate less air resistance ... it generates more air resistance because you not only have air friction on the nose but also the spinning sides "rub" against the air way more than a bullet that is not spinning. Bullet spin slows with air friction at about the same rate as velocity loss from air friction. That's why some bullets lose stability down range and begin to tumble. Ballistic coefficient is the number assigned to bullets based on their ability to overcome air friction. Shape, length, and diameter dictates B/C. In theory, a bullet with a B/C of 1 would have zero air friction. Good rifle bullets often have B/Cs of .5 or better. Handgun bullets are not very aerodynamic and typically run .2 or less.

I agree .... bullets continue to increase in velocity with longer barrels. There's not a handgun barrel long enough to defy this principal, although the rate of increase may only be token after a given length. Once a bullet starts into the bore and is "engraved" by the rifling, it doesn't take much pressure to continue pushing it down the bore. The rate of increase is strictly dictated by the quantity of powder and its burn rate. A slow burning powder such as W-296 will peak in pressure after the bullet has traveled a couple inches but it takes 12 or more inches of bore before the powder is totally burned up. So ... slow burning powder creates way more muzzle velocity than fast burning powder with the same initial chamber pressure because the pressure doesn't drop off as fast. Fast burning powder like Bullseye will reach peak pressure before the bullet leaves the case so it really kicks the bullet in gear initially. Bullseye is completely burned up in 2 inches of bore but there is still plenty of barrel pressure to accelerate the bullet for at least 16" of bore.

I ran a 38 Special +P chart in QuickLOAD just for grins. I used 4.9 grains W-231 powder and a 158 gr jacketed bullet, which is one of my pet loads. Rather than explain all the stuff, here's the chart. Note: The red numbers on the left side are chamber/barrel pressure and correspond to the red trace on the chart. The black numbers at the bottom are barrel length (inches of bullet travel). The Blue numbers on the right are velocity in fps and correspond the the blue trace. On the computer, there is a crosshair that you can place on either trace to accurately measure velocity or pressure at any barrel length. You should be able to get a fair idea of what happens inside the chamber and bore.



As you can see, pressure peaks very quickly and reaches a max of 18,259 psi in 1.244" of bullet travel. Pressure then starts dropping off but at 8" of bullet travel, there is still 2100 psi pushing the bullet. Velocity increases very quickly in the first few inches and continues to increase at a slower rate until the bullet exits the muzzle. The powder is 95% burned at 2.068" of bullet travel and totally burned at 3.218" of bullet travel.

The B/C gap in a revolver does indeed lose a significant amount of pressure (not shown in the above chart). The wider the gap, the greater the loss. On an average, you will lose about 1.5% of muzzle velocity for each thousandth of an inch of B/C gap. That means an average revolver with a .005" B/C gap will lose about 7.5% of its muzzle velocity when compared to a non-vented barrel of the same length shooting the same ammo.

Here's the kicker .... pistol barrels (semi-autos) lose very little pressure because the breach doesn't unlatch until the bullet exits the muzzle. Pistol barrels are measured from the muzzle to the breach face so a 5" pistol barrel only has about 4" of actual bore. Revolver barrels are measured from the muzzle to the cylinder face so when you add a couple tenths of an inch for the throat, a 5" revolver barrel will have 5.2" of actual bore. Turns out, when the same ammo is fired from a 5" revolver and a 5" pistol, the chronographed velocity will be almost identical. Look at the charts and find the velocity at 5.2" (921 fps). This is what the velocity would be in a non-vented barrel with 5.2" of actual bore. Now look at the velocity at 4" (857 fps), which is what it would be in a 5" pistol barrel. So ... using this particular load, a 5" revolver will actually chronograph at about 857 fps +or- a little for B/C gap differences. In this case, the revolver actually looses 64 fps due to the B/C gap, which is about 7%.

The pressure loss from the B/C gap happens primarily when the base of the bullet is flush with the rear of the barrel. After the bullet starts traveling down the bore, a little more pressure is lost but not a significant amount. Hope this helps and is not too confusing.

edlmann

[SIZE="4"]There is a theoretical possibility that the direction of the rifling might tend to either tighten or loosen the barrel every time the gun is fired. I am skeptical.

For about 40 years (say, 1920ish to 1960ish) Chrysler Corp vehicles had left hand threads on the lug nuts on one side to prevent the nuts from loosening themselves. If you didn't know the secret, it could drive you crazy. After the 40 years or so, someone at Chrysler noticed that the ditches were not full of Chevys and Fords whose wheels had fallen off. In the words of the late Emily Litella, "Never mind".[/SIZE]

thebiggiantkevin

Those are actually very god questions, and answers.
I agree with everything above except one of Iowegans comments. There may actually be a handgun barrel long enough to defy the principal...

JRow

That's exactly what I thought

Thanks for the info - JR

support_six

Right side of a Dodge Dart – "Righty - Loosy"! It was really hard getting that first lug nut off after stood on the lug nut wrench!

firescout

I think that some people erroneously believe that a bullet in flight has lift, due to the sight-in trajectory charts. They show that the close (within 100 yds. from the muzzle, if sighted dead-on at 100 yds.) bullet trajectory is above the sight line. But that is due to the need to *tip* the muzzle up slightly (from the sight line) so that the bullet will impact the desired point *way out there*. If the bore line was aligned with the sight line, the bullet trajectory would start out below the sight line, and would be progressively lower as it travelled downrange from the muzzle.

Sometime, we should discuss the true meaning of the term *point blank*...

Iowegan

firescout, BronxBoy referred to the Bernoulli effect, which is a phenomenon where low pressure resulting from air passing over on object creates a lifting action. This actually happens with bullets but it happens on the bottom too with an equal and opposite force. The effects are self canceling, leaving just gravity to pull the bullet down. The Bernoulli effect doesn't work on round objects but it does work well on airplane wings.

Yes, I agree ... many people do think bullets rise because of what they see in trajectory charts. Here's another one .... lay a centerfire handgun on the floor then take a yardstick and place the edge on the top of the front and rear sights . Stand up and look at the difference in angle. The yardstick will actually be pointing several degrees above bore line. When you aim at a target, the sights are pointed directly at the target but the bore is pointed several inches below the target. This would make one think bullets must rise but indeed what happens is recoil makes the muzzle rise to track the line of sight.

Last but not least .... a couple years ago I got into a heated discussion on another board about bullet velocity. Seems this guy was convinced .... bullets gained speed as they went down range and nothing I could say would change his mind.

edlmann

[SIZE="4"]The near one or the far one? [/SIZE]

Iowegan

Here's how Wickipedia defines "point blank range".

In external ballistics, point-blank range is the distance between a firearm and a target of a given size such that the bullet in flight is expected to strike the target without adjusting the elevation of the firearm. The point-blank range will vary with the firearm and its particular ballistic characteristics, as well as the target chosen. A firearm with a flatter trajectory will permit a nearer minimum and further maximum point blank range for a given target size, while a larger target will allow for a longer point blank range for a given firearm.[1]

In forensics and popular usage, point-blank range has come to mean extreme close range (i.e., target within about a meter (3 ft) of the muzzle at moment of discharge but not close enough to be an actual contact shot)[1]. Gunshot wounds from point-blank are identified by extensive powder burns as well as tissue damage from perforation by unburned grains of gunpowder

JRow

Wouldn't that make a .357 shoot higher than a .38 in the same gun? My wife and I are both more accurate with a .357 (smaller group and better centered).

A Patriot

JRow, you may be refering to barrel lag time, in which a bullet with a high velocity and the same bullet with a low velocity -- with the same point of aim -- will have two different points of impact, due to the difference in the length of time the bullet is in the barrel, in relation to the muzzle's rise under recoil.

I believe there are some exceptions -- notably with the .44 Special -- though I've experienced it with my .44 S&Ws.

Iowegan

JRow, Typically it does. Recoil starts to happen when the powder first gets ignited ... well before the bullet exits the barrel. Recoil coupled with the pivot of your wrists or elbows is what generates muzzle rise. The things that affect muzzle rise are:
1. The weight of the bullet, heavier bullets result in more muzzle rise.
2. The length of time the bullet spends in the barrel. Higher velocity bullets spend less time in the barrel and result in less muzzle rise. Longer barrels have more muzzle rise because it takes longer for bullet travel.
3. The weight of the gun. Heavier guns have less muzzle rise. Muzzle heavy guns have less muzzle rise.
4. The power level of the load. More powerful loads will have more muzzle rise.
5. Jet blast. Short barreled guns (think snubby) release the bullet from the muzzle while chamber pressure is still very high. The release of high pressure causes a considerably stronger jet blast than a longer barrel and will make the muzzle rise more (see the chart in post #4 of this thread).
6. Grips. The shape of the grips can change the pivot point for your wrists and result in either more or less muzzle rise. Also related ... how firm you grip the gun and how tight you lock your elbows will have a dramatic affect on muzzle rise.

Some of these conditions will offset each other. Example: a high velocity, high power, light bullet .... high velocity = lower rise. Light weight bullet = less muzzle rise, but the powerful load = more muzzle rise. When you factor everything together, this combination will shoot lower than another load of equal power. Another example: a long barreled gun that is muzzle heavy will offset the effect of longer bullet travel time.

I find when I shoot 357 Mags, I tend to lock my elbows and wrists tighter than when I'm shooting 38s. Assuming the same gun and same bullet weight, my POI is very close to the same with both loads, even though the 357s are way more powerful. If I grip the gun more firmly with the 38s, I'll shoot about 6" low. So ... even though there are several technical reasons for more/less muzzle rise, the way you grip the gun makes more difference than anything else; however, no matter how strong you are, the muzzle will still rise some.