Bullet trajectory


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blackops
July 10, 2009, 03:24 AM
Correct me if I'm wrong please. I have a 270 and at 100yds my bullet should be 1.5 inches high, this in turn means at 200 yds I'm dead on. So the bullet out of the barrel actually rises before it starts to drop correct? With the rifeling of the barrel it gives the bullet that tight rotation lifting the bullet right or wrong?

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Tim the Enchanter
July 10, 2009, 03:33 AM
Gravity acts on the bullet as soon as it leaves the muzzle, and the bullet begins to drop. Your scope is tilted down and looking further down than the bullet will actually drop at a given range, creating the illusion of lift.

See:
http://www.pyramydair.com/blog/images/10_19_07_Figure_2.jpg
and
http://www.pyramydair.com/blog/images/10_19_07_Figure_3.jpg

MTMilitiaman
July 10, 2009, 05:01 PM
No. As mentioned previously, gravity begins acting on the bullet as soon as it leaves the muzzle. Therefore, if the muzzle is level, the bullet will never go above the line of sight.

Sighting in a rifle is done by inclining the muzzle such that the bullet rises above, then drops back down to, the line of sight at a predetermined range. The arc of the trajectory then brings the bullet highest above the line of sight around midway between the shooter and the sight-in distance. This is referred to as the 'midrange trajectory.' Once the bullet reaches this point, it will drop continuously throughout the rest of its flight-dropping to the line of sight at the sight-in distance, and then exponentially below the line of sight from there on. Because of this, the bullet crosses the line of sight twice--once rising above the line of sight due to inclination of the bore, and again coming down to the line of sight at the sight in distance. The first time the bullet crosses the line of sight is usually between 20 and 30 yards for most centerfire rifles, depending on velocity, and the height of the optic above the bore. This is why when sighting in our centerfire rifles, it is standard practice for us to sight in for windage and elevation at 25 yards, then make corrections as necessary at the desired sight in distance. This works quite well with most hunting rifles and optics sitting ~2.5" above the bore (center of objective lens to center of bore). The higher the optic is above the bore, the further down range the bullet will initially cross the line of sight. For example, the 4x Trijicon RCO mounted on the M16A4 sits higher above the bore than most rifle/optic combination. This combined with a desired zero range of 300 yards means that the optics are initially sighted in at 36 yards. The BZO (Battle ZerO) is then confirmed on pop-up targets at 300 yards.

Rifling puts a spin on the bullet because modern spitzer (pointed) style projectiles are heavier towards the base than the nose. Therefore, they naturally want to fly base forward. Rifling imparts a spin on the projectile to overcome this tendency. The longer a projectile is, or the more dense the medium is that it is traveling through (air, tissue, water, ect) the faster it must be spun to be stable. Think of it like a top. The longer a top is, the faster it must be spun to remain stable. Once it drops below the necessary rotational velocity, it quickly loses stability. The same thing can happen to a bullet when it drops below the rotational velocity necessary for it to remain stable in a given medium, at which point it tumbles or "keyholes." While rifling can have a slight effect on linear velocity as well, its effect on external trajectory is minimal.

Think of it as throwing a football down field. Few people possess the arm strength and coordination to throw a laser pass dead-on much past 15 yards. To get the ball further down the field, it must be thrown upwards at an arc so that it falls to the desired target at a specific range. Imparting a spin on the football isn't quiet as necessary as the football is more balanced than most rifle bullets, but it still helps the ball fly more aerodynamically.

waterhouse
July 10, 2009, 05:05 PM
here is another picture which may help. The bullet start out below the line of sight, crosses the line of sight at the first zero, and then continues along its arc until it falls to the line of sight again, which is generally where people say the rifle is "zeroed."

http://img.photobucket.com/albums/v601/waterhouse/02-trajectory.jpg

rcmodel
July 10, 2009, 05:11 PM
+1 to all the posts above.

The only way a bullet could generate lift is if it were airfoil shaped like a wing, and not spinning.
Thus presenting a constantly changing round profile to the air that cannot generate lift in an upward direction.

If the bullet yaws during flight, and all bullets do to a certain extent, it would generate lift in the direction of the yaw.

But again the bullet is spinning thousands of times per second, and the yaw angle is constantly changing around the 360 degree rotation.

rc

iiranger
July 10, 2009, 05:28 PM
an article that Weaver (or TX) used to include with scopes...

What has "stuck in my head" now these 5 decades:

You survey 100 yards exactly level. Put the .22 rimfire rifle on the bench at 48 inches, say. Bore sight. Target center also at 48 inches. Load one long rifle and fire. Bullet will strike 12 inches below the center. Move back to 125 yards and the bullet will strike 36 inches low.

While the bullet starts to fall as it leaves the muzzle, you point the "line of bore" so high above the target that the bullet will fall into point of aim. And you use reference points on the barrel, called sights, since you cannot look through the barrel while firing... !!

Uncle Mike
July 10, 2009, 05:31 PM
If the bullet yaws during flight, and all bullets do to a certain extent, it would generate lift in the direction of the yaw.

Huh?......:scrutiny:

rcmodel
July 10, 2009, 05:38 PM
Like sticking your hand out the car window going 70 MPH.

If you yaw, or tilt your hand up, it goes up.
If you tilt it down, it goes down.
If your hand is spinning 300,000 RPM like a bullet, it pretty much doesn't go anywhere though.

See this about yaw:
http://www.exteriorballistics.com/ebexplained/5th/40.cfm

rc

Uncle Mike
July 10, 2009, 05:49 PM
With the rifeling of the barrel it gives the bullet that tight rotation lifting the bullet right or wrong?

Does your hand drill try to 'fly' out of your hand whilst it's spinning?....
Does your drill press scoot across the floor when in use.

The 'spinning' of the projectile does not create any lift, the 'spin' only offers the gyroscopic forces that stabilize the said projectile.

As the knuckle ball thrown with no spin rises and falls unpredictably the two finger fast ball that has spin, stabilizes and offers the pitcher control ability...accuracy if you will.

All the above explanations in the other posts are spot on... do you understand.....y? or n?:)

The next time your out with the water hose, hold the nozzle parallel to the ground and squirt away... notice where the water hits the ground... now angle the nozzle slightly upwards.... notice how much further away the water is hitting the ground?

Same principle with the projectile.;)

:D

cbrgator
July 10, 2009, 05:53 PM
Bullets really spin at 300,000 rpm? Hmm... guess you learn something everyday! That's very cool.

Uncle Mike
July 10, 2009, 05:57 PM
Like sticking your hand out the car window going 70 MPH.

Right... but your hand is flat and isn't creating any lift, per sey, but is being deflected perpendicular to the direction of the oncoming wind... kind of like a sheet of plywood in a wind storm.

If you do not tilt the hand, or plywood from the parallel position in relation to the direction of the oncoming wind, it will remain in that position because there is no lift being created, an foil on the other hand will create lift in all but a 'stalled' position.

To create lift your hand would have to exhibit the shape of an airfoil, creating pressure differentials which allow for increased pressure on the bottom and decreased pressure on the top of the foil, respectively.

:D

rcmodel
July 10, 2009, 06:33 PM
Bullets really spin at 300,000 rpm?Some spin even faster then that.

Example One: In a 1:12″ twist barrel the bullet will make one complete revolution for every 12″ (or 1 foot) it travels through the bore. This makes the RPM calculation very easy. With a velocity of 3000 feet per second (FPS), in a 1:12″ twist barrel, the bullet will spin 3000 revolutions per SECOND (because it is traveling exactly one foot, and thereby making one complete revolution, in 1/3000 of a second). To convert to RPM, simply multiply by 60 since there are 60 seconds in a minute. Thus, at 3000 FPS, a bullet will be spinning at 3000 x 60, or 180,000 RPM, when it leaves the barrel.

Example Two: What about a faster twist rate, say a 1:8″ twist? We know the bullet will be spinning faster than in Example One, but how much faster? Using the formula, this is simple to calculate. Assuming the same MV of 3000 FPS, the bullet makes 12/8 or 1.5 revolutions for each 12″ or one foot it travels in the bore. Accordingly, the RPM is 3000 x (12/8) x 60, or 270,000 RPM.

Then, consider a .223 WSM, with a 1/10 twist, and a 45 grain bullet going 4,100 FPS.
I'll let you do the math. :D

rc

John Parker
July 10, 2009, 06:44 PM
Well RC, it's not quite that simple. Not only does the bullet begin to drop when it leaves the muzzle, it also begins to slow down...neither trajectory nor acceleration are static with objects moving due to an external, non-constant power source.

Neener neener!

Uncle Mike
July 10, 2009, 06:45 PM
Then, consider a .223 WSM, with a 1/10 twist, and a 45 grain bullet going 4,100 FPS.
I'll let you do the math.

What is... 295,200 RPM? I dunno! It's damn fast! I know that hehehe

:D

rcmodel
July 10, 2009, 06:54 PM
Well RC, it's not quite that simple.I know how simple it is.
I was shooting 1,000 yard high-power for 5th. Inf AMU in 1968.

We pretty much had to understand exterior ballistics to get on paper the first day.

Short of writing a book, it's a little much to explain to someone who ask about bullet lift being caused by rifling spin in the OP.

Neener neener to you too!

rc

Uncle Mike
July 10, 2009, 07:02 PM
hehehe.... bottom line is... the bullet starts to head for the center of the earth AS SOON AS it leaves the barrel.;)

We angle our barrels upward to get that magical extra distance.... water hose thing again, so that is why the projectile crosses your line o' sight(which is as straight as a laser from our eye to infinity) twice. Once on the way up, and once on the way down.;)

:D

gunnie
July 10, 2009, 08:10 PM
back to the OP's question...

...I have a 270 and at 100yds my bullet should be 1.5 inches high"...

this will vary in relation to the scope center line distance from the bbl centerline.

the bullet path will be higher @150 yards in relation to POA, and then start to slow/drop enough to cross the line again.

as above, it starts to drop upon leaving the bbl, it is the downward angle of the scope crosshairs, not the bullet rising.

gunnie

Uncle Mike
July 10, 2009, 08:29 PM
Correct me if I'm wrong please. I have a 270 and at 100yds my bullet should be 1.5 inches high, this in turn means at 200 yds I'm dead on.

Provide us with the ammo you are using, bullet weight and ballistic coefficient also, the length of the barrel which you will fire this load from and the zero distance and we can provide you with various POI's and trajectories.

You can have some fun at this site:http://www.norma.cc/default.asp?Lang=2#

go to ballistics US. Play with this computer, it is a good visual trajectory form.

:D

John Parker
July 10, 2009, 08:34 PM
I was shooting 1,000 yard high-power for 5th. Inf AMU in 1968.

Ah, the ol' Army Marksmanship Units. You know, I learned more about the 240B from those guys than anyone else. Pretty amazing!

Whitman31
July 10, 2009, 10:52 PM
This may be a stupid question, humor me...

I've hear rumor that a bullet fired while the muzzle is very close to the ground will actually rise some. Suggesting that the muzzle blast reflects off the ground and provides a little upward push. Is this just a myth?

tactikel
July 10, 2009, 11:05 PM
A bullet would be infront of any muzzleblast, and long gone before any gas/shockwave could bounce and deflect it- IMHO

RC has forgotten more than most of us will ever know about rifles and shooting ;) 200k-300k rpms are why some varmint bullets will fly apart when fired from a barrel with too fast a twist.

iamkris
July 11, 2009, 10:12 AM
Technically, a cylindrical object that is spinning counterclockwise into a relative wind will generate a very small amount of lift. Most first year college physics classes will have an experiment with a paper towel roll that demonstrates this.

The "top" of the roll is moving faster than the bottom of the roll into the relative wind...that means you'll have lower pressure on the top vs the bottom...thus lift.

That said, it is minute for a bullet and of little consequence.

See...that bachelors in mechanical and masters in aerospace engineering came in handy after all! (I'm not an engineer anymore).

Hud
July 11, 2009, 10:49 AM
Go here & download this Canadian manual on ammunition & ballistics. Good info.

http://stevespages.com/zip/canadian_b-gl-306-006fp-001%20-%201_june_1992.zip

gunnie
July 11, 2009, 04:34 PM
Hud,

good stuff there!!!

gunnie

Quilbilly
July 11, 2009, 05:20 PM
Is there anything RC does not know?

blackops
July 11, 2009, 05:40 PM
Provide us with the ammo you are using, bullet weight and ballistic coefficient also, the length of the barrel which you will fire this load from and the zero distance and we can provide you with various POI's and trajectories.

.277 Sierra HPBT 135 GR
Ballistic Coefficient .488
20" barrel
zero distance 200yds

Tarvis
July 11, 2009, 06:32 PM
Gravity acts on the bullet as soon as it leaves the muzzle, and the bullet begins to drop
If you point the rifle at the sky and fire it, is it not true that the bullet is in fact rising? After all, the OP did ask if the bullet rises before it drops; in any instance where the muzzle is pointed "above level with the ground" (or however you are supposed to phrase that), yes, the bullet does rise before it drops. The idea that a bullet momentarily creates lift or temporarily defies the laws of physics... you get the idea.

What if someone created a bullet that would turn sideways after leaving the muzzle, causing the rifling grooves to create lift like a baseball? Then it really would rise... someone start designing a tiny gyroscope!

Rifling puts a spin on the bullet because modern spitzer (pointed) style projectiles are heavier towards the base than the noseYou meant to say because the rifling circles the bore, right? And what about wad cutters? Or are you trying to say that rifling is used so spitzers can be used? Sorry, I don't mean to pick on you or anything; it seems like you were making the point that rifling was invented for use with spitzer bullets.

blackops
August 2, 2009, 01:46 AM
I love how people take time to make a post and say something that everybody understands, but there is always one guy who wants to pick out the smallest imperfection as far as grammar or reasoning and act is if they are perfect.

bigalexe
August 2, 2009, 05:57 PM
I believe the formula for Bullet RPM is the following:

FPS*12*60*Twist=RPM

A breakdown is as follows:

FPS*12 gives you Inches Per Second, then Multiplied by 60 is Inches Per Minute. The Twist of a rifle is usually stated as follows 1/7 so thats 1 twist in 7 inches. Now what we needed to do was divide the inches of travel by the distance it takes a bullet to do a full rotation. Whats handy is that multiplying by a fraction is actually a division problem, so multiplying your inches per minute value by the fraction 1/7 actually divides by seven. So the result is you answer.

For more fun take your bullet diameter (ever so handy its the caliber) and multiply by Pi (3.14) and then multiply that value by your RPM. That would tell you how far sideways your bullet would travel if it were rolling on a surface instead of flying straight. This serves no purpose but: Its fun to find out that a .50 caliber bullet at 2000 FPS out of a 1/7 twist barrel would travel nearly 5 miles sideways in a minute.

WNTFW
August 2, 2009, 06:15 PM
Bigalexe,
Never thought about the sideways travel thing. It's not useless if it will make a good excuse for those pesky "Flyers". Just kiidding really, I own all my flyers.

WNTFW
August 2, 2009, 06:19 PM
Black Ops: "I love how people take time to make a post and say something that everybody understands, but there is always one guy who wants to pick out the smallest imperfection as far as grammar or reasoning and act is if they are perfect."

I know what you mean. I guess gravity is acting on the bullet at all times, not just after leaving the barrel. It becomes about the semantics.

content
December 4, 2009, 07:12 PM
Hello friends and neighbors // Will rotating the position of the front sight from 12:00 to say 9:00 affect trajectory?

It seems the rifling is used in conjunction with the sights for a straighter shot.

wishin
December 4, 2009, 07:59 PM
The only way a bullet could generate lift is if it were airfoil shaped like a wing, and not spinning.


I wish I'd had all you guy's to back me on a different thread. I finally gave up on trying to convince a few hard heads of that fact.

benEzra
December 4, 2009, 08:20 PM
The only way a bullet could generate lift is if it were airfoil shaped like a wing, and not spinning.
Well, it could generate a teeny bit of lift it it were traveling sideways or at a significant yaw angle, but not while traveling point first to the relative wind (which it does to a tiny fraction of a degree).

BTW, to the OP's question, think of throwing a baseball 75 feet. The ball does not start out moving level to the ground and then rise after leaving your hand. You have to throw it at a significant upward angle to lob it any distance at all, and it never climbs above that initial trajectory. It's exactly the same with a bullet fired from a gun.

AKElroy
December 4, 2009, 08:30 PM
I hope this is not a hijack, but I have a related question. I have a nikon BDC on a .243. Ordinarily, I would sight this rifle 1.5" high @ 100, but with the BDC, I have a 100, 200, 300 & 400 mil-dot circles in the reticle. Should I zero dead on @ 100 with this scope?

Eaglecreekbrewer
December 4, 2009, 09:01 PM
This is one thing I still remember from hs physics.

Velocity has no effect on gravity.

Problem:

Rifle barrel is level
Ground is level
Bullet is dropped by hand at barrel level, at the exact instant a bullet exits the barrel at 3000fps. Which bullet will hit the ground first?

Answer: They will both hit at the same time, even though one will be at your feet and the other will be thousands of feet away. The force of gravity is equal on both bullets.

Art Eatman
December 4, 2009, 11:14 PM
AKElroy, that should work IF the mil dots correspond to the trajectory of your bullet. If they don't, you either have to memorize what they actually represent or ignore them--and sight in for the commonly-used 200-yard zero.

Mr_Pale_Horse
December 5, 2009, 08:52 AM
Yawing and/or pitching of bullets - It happens, but the effects are not perceptible at ranges as short as a few hundred yards, for 2 reasons.

1. The bullet is hyper-sonic. Suffice it to say, airflow models at the surface of the bullet are not those witnessed in a physics class demonstration with a TP tube.

2. In real time, bullets cover alot of ground fast. 200,000 rpm sounds like alot, but a bullet can cover 100 yards in ~100 milliseconds, or .1 seconds, or 333 revolutions.

At 1000 yards, 168 grain bullet with a nominal .5 ballisctic coefficient, reaches that range in 1.461 seconds, at least according to the exterior ballistics program I have. Not nearly enough time for a pitch/yaw induced rise to be perceptible. Moreover, it is still supersonic at that range.

Bart B.
December 5, 2009, 09:56 AM
Mr. Pale Horse sez:At 1000 yards, 168 grain bullet with a nominal .5 ballisctic coefficient, reaches that range in 1.461 seconds, at least according to the exterior ballistics program I have. Not nearly enough time for a pitch/yaw induced rise to be perceptible. Moreover, it is still supersonic at that range. It'll be supersonic at 1000 if it's a 30 caliber one leaves at least 2600 fps. The military M852 7.62 NATO match ammo with this bullet oft times didn't shoot fast enough from 22-inch barrels on M14's; they went subsonic at about 900 yards and accuracy suffered.

Casefull
December 5, 2009, 12:42 PM
There are two kinds of knowledge. Head kind and experiece kind. When the two are melded together and refined (ie the crap is thrown out of the equation) then you have rc's knowledge. Thinking and believing something to be true is not the same as doing it.

X-Rap
December 5, 2009, 12:54 PM
Sometime spliting hairs that have already been split leaves very little knowlege to be gleaned but makes the splitter feel good about themselves. Some people need that.

AKElroy
December 5, 2009, 02:01 PM
AKElroy, that should work IF the mil dots correspond to the trajectory of your bullet.

Thanks Art. I cannot break years of practice, so I will be sighting for a 200 yrd zero & ignoring the mil-dots. Should have saved $10.00 on the scope & gone with the standard reticle.

Offfhand
December 5, 2009, 02:34 PM
While everybody is busy confusing each other be sure to toss in the coriolis effect, that always helps.

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