Physics of shooting a rifle

[tangolima]

Yes, the logic indicates that the recoil starts when the bullet starts moving but most footage can lead to misconceptions.
I found the clip with the AR bolt interesting too, it clearly shows two impulses: a small one before the the bolt is fully to the rear and another larger one when everything “bottoms out.”

Thank you for taking the time to reply.

The 1st impulse is the "normal" recoil force due to the bullet accelerates down the bore. The 2nd impulse is the result of the bcg + buffer bottoms out on the buffer tube. The first one affects how the rifle shoots, and the second one affects how the shooter feels, although I have met people who insist both affects the rifle's (not the shooter's) accuracy.

It is generally believed that auto loaders kick less. Clearly it is the 2nd impulse we are concerning. Can a semi auto kick worse? Can a light recoiling gun, such as an AR, kicks even lighter? And how?

A new conejo hole perhaps.

-TL

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[rickyrick]

I slowed down the clip with the AR bolt to 25% and you can clearly see that the first recoil impulse has started the rifle movement before the gas impulse is apparent at the bolt carrier vents.

 

[JohnKSa]

Yup. Has to be. When the bullet starts moving, so does the rifle. The bullet has been moving for several inches before it passes the gas port and the gas can start to unlock and operate the bolt.

Here's my take on what happens when firing an AR15.

• Bullet starts moving and rifle (and all its parts) starts recoiling.
• Bullet passes gas port.
• Gas moves through gas port, into the gas tube and pressurizes the gas system.
• Pressurized gas system begins accelerating the bolt carrier backwards. The rearward acceleration of the bolt carrier imparts a forward "recoil" impulse on the rifle. Not enough to overcome the overall motion of the rifle to the rear, but it reduces the recoil velocity of the rifle by some small amount.
• Bullet exits the bore.
• Bolt carrier motion continues due to momentum and unlocks the bolt.
• Entire bolt continues moving to the rear to extract/eject the empty cartridge.

 

[davidsog]

When the bullet starts moving, so does the rifle.

Yes. The laws of physics must be obeyed.

The recoil discussion is a very good example of how those effects can be obscured especially when dealing with different Frames of Reference.

I cleaned up the Sierra Engineers diagram of the Top Down view of what going on with the trajectory of a Bullet. As the diagram is presented, it is kind of busy as it shows several different things all at once. The Trajectory of the bullet relative to the Air, The Trajectory of the Bullet relative to the Ground, The velocity of the bullet relative to the Air, and the Velocity of the bullet relative to the ground. All of those on one graph can be confusing when trying to separate the concepts and effects.

First we have the Velocity of the Bullet RELATIVE to the Air. When the bullet is in the gun, it is an object of the ground and its velocity relative the to Air is ZERO. The moment it leaves the forces acting upon it become subject to the laws of aerodynamics.

The bullet picks a path that equalizes the forces acting upon as best it can. It can only equalize three axis much like a glider.

That equilibrium point results in the bullet achieving ZERO velocity Relative to the Air and just like any other object in flight, it moves with the mass of air.

Reinforcing this point, Sierra Engineers diagramed the Velocity of the Bullet relative to the Crosswind. The bullet exits the muzzle, torques around the CG balance the forces in three axis, and the bullet moves to a relative velocity of zero with the Crosswind.

 

[stagpanther]

So David; as you know I 100% agree with the air mass relativity as the principal frame of reference for determining the trajectory of a bullet between muzzle and target.

In practical terms, though, how can the average shooter easily and quickly apply a construct to this that equals or exceeds the accuracy of the present systems used to estimate aim adjustments? That is what I have trouble grappling with.

 

[davidsog]

So David; as you know I 100% agree with the air mass relativity as the principal frame of reference for determining the trajectory of a bullet between muzzle and target.

In practical terms, though, how can the average shooter easily and quickly apply a construct to this that equals or exceeds the accuracy of the present systems used to estimate aim adjustments? That is what I have trouble grappling with.

No, we are not inventing new math. The current math approximates bullet behavior well enough.

We are simply understanding what is going on with our bullet and eliminating conjecture as well as false conclusion based upon ground frame reference observation.

That being said, Military bullets are designed with math and measurements that incorporate much higher level of performance math including aerodynamics. Sierra engineers relate this and state the equipment exists to measure the exact angles/forces on the bullet. It is just expensive and nobody has ever bothered to do it for civilian bullet designs. They state that one day they hope to do this with their bullet designs as the equipment becomes more available to the public.

 

[rickyrick]

I’m learning a lot on this thread, I’ve always just shot and haven’t put much thought into what happens to the bullet except for the drop, and layman type information.

I’m curious as to how much the muzzle blast affects the trajectory of the bullet. It’s clear that for a short period, the gases overrun the bullet. I’m making an assumption that the “target crown” has something to do with this.
Additionally, how do things like compensators and suppressors factor into this?

 

[davidsog]

I’m making an assumption that the “target crown” has something to do with this.

We were taught the crown is extremely important in accuracy. Any damage to it will ruin a weapon's and a properly shaped one makes all the difference. In our Sniper rifles, we always used bore guides, solid brass cleaning rods, Jags, and always went from Chamber to Crown removing the patch before carefully withdrawing the empty rod. All to protect the crown. The most damaging thing you can do to a rifle is use a segmented steel rod in cleaning. Which always made me shake my head as that is the standard Army issue cleaning kit for the Regular Army, lol. Joe was always jamming that thing down the flash suppressor.

Suppressors main effect is on barrel harmonics. It will generally effect your POI and require you to re-zero with a suppressor. A properly mounted and designed suppressor acts as a dampener and can improve accuracy. A poorly matched set up can ruin it and amplify oscillations. We always re-zeroed with a suppressor, remembered the windage/elevation changes from unsuppressed, and kept the rifle/suppressor as an issued together. We did not "mix and match" suppressors and weapons. Each rifle and suppressor combination is different as to its effects on accuracy. Most important thing is having a well made suppressor and properly designed mount.

The design of the suppressor can "maybe" impart some velocity changes on the bullet for the most part I have never seen it. The suppressor design of the interior baffles has led to this claim. It seems plausible, as the entire purpose of a suppressor is to create turbulence to break up the normal shock waves of the expanding gas which create the "crack and boom" we heard from a bullet/explosion. Turbulent flow is high energy flow and is responsible for the majority of lift production in an airfoil. It is an interesting question.

I’m curious as to how much the muzzle blast affects the trajectory of the bullet.

My understanding is muzzle blast does not effect the bullet in any significance. Once the gas leaves the muzzle it becomes spherical in shape and very quickly loses velocity to mach 1. Most bullets are traveling faster than mach 1. Additionally, that energy is already on an established axis opposing drag with little opportunity to impart a side force. In a subsonic bullet, muzzle blast would impart some effect I would imagine as it cannot outrun the expanding gases.

 

[davidsog]

The Sierra Engineers say:

When a bullet exits the muzzle of a gun, it immediately begins some angular pitching and yawing motions which have several possible causes including the crosswind.
These angular motions are small, cyclical, and transient.

The "small, cyclical, and transient" means they cancel themselves out for the overall picture and are gone.

 

[rickyrick]

I’ve seen where the bullet is doing some pitching and yawing before it’s stabilized, some much more than others. Pretty interesting.

 

[tangolima]

Gas exiting muzzle over takes the bullet momentarily. It is indeed a critical moment, even with perfect crown.

99.9% of a projectile's flight is with a dominant head wind, and therefore it is an indispensable component in stabilization. With head wind suddenly gone and reversed to tail wind, even momentarily, something can easily go wrong.

I have experimented seating bullet backwards. The bullet tumbles every time. Without going into details, a spin stabilized projectile requires center of pressure (CP) be ahead of center of gravity (CG). A tail wind reverses that. The bullet becomes unstable for fraction of a millisecond. Any minute imbalance of air flow, a cross wind, a less than perfect crown, can impart yaw and pitch on the bullet that requires some iterations in bullet's posture in flight to correct.

Not much can be done to avoid this I am afraid. A good crown is of course essential. Some suggested flash cone / can to shield the bullet from cross wind. Its efficacy is questionable unless the cross wind is super strong, such as waist guns on a B17. How about taking away the gas before the bullet exits muzzle? Muzzle device won't work, ported barrel may.

This is for spin stabilized projectile. Situation is more hopeless in fin stabilized airgun pellet. That's why the barrel is rifled even though a pellet does need that for stabilization.

-TL


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[davidsog]

I’ve seen where the bullet is doing some pitching and yawing before it’s stabilized, some much more than others. Pretty interesting.

It is interesting. The concept of those pitch and yaw movements having an overall effect of zero is weird. It is counterintuitive but if you understand vector math then the vectors cancel each other out. I also think it difficult to grasp how quickly the bullet achieves equilibrium on 3 axis like a glider. Sierra Engineers relate that the stabilization occurs in the heaviest, most powerful bullets in 1/10th of second with most bullets achieving it much quicker. The order of these effects are very small.

I have experimented seating bullet backwards.

Current Ballistic math is only approximates behavior in just a few basic shapes. The most common, G2 and G7 refers to just two basic shapes.

https://kestrelmeters.com/pages/g1-g7-ballistic-coefficients-what-s-the-difference

Reversing your bullet is not one of those shapes and represents being a bullet test pilot so I am not surprised if you observe unusual behaviors. In fact, I would think your bullet would tumble and be unstable as you have changed the design relationship of the CP to CG.

 

[Vinootz]

On page 1 of this thread some were calculating physics formulas. Why not just point and shoot a rifle you’re comfortable with. And if democrats catch wind of that, they may start drafting gun control legislation that requires people to prove they understand physics formulas.

 

[davidsog]

On page 1 of this thread some were calculating physics formulas. Why not just point and shoot a rifle you’re comfortable with. And if democrats catch wind of that, they may start drafting gun control legislation that requires people to prove they understand physics formulas.

Just use "The Spontaneous Generation Theory" of Ballistics....

https://youtu.be/AngYmfS7pFI?si=K7O_6FWGI2Biqdso

 

[tangolima]

Another rabbit hole on wind deflection.

Target at 300yd. Wind flag at firing line and every 100yd. The one at 300yd is in scope's FOV so it is handy. The other ones I have to scan with left eye. Wind is varying at different distances. Which flag should I pay most attention to and why?

Say I got 2" wind deflection at 200yd. Same wind up to 200yd, but no wind from 200yd to 300yd. The wind deflection at 300yd is still 2"?

-TL

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[tangolima]

Nobody home apparently. Here are my inputs.

A sideways force is asserted on the bullet as result of cross wind. That force accelerates (velocity keeps increasing) the bullet sideways. More time the bullet is in cross wind, more it is deflected by the wind.

Following this mechanism, it is the wind at the firing line that matters the most, and the wind at the target has little or no effects.

Varying wind down the range is difficult. Different shooter has his own method. I use weighted average. Wind at firing line as weighting of 4, mid range 2, target 1. The weighted average is used for initial dope. Fire and adjust.

The wind accelerates the bullet sideways. Even if the wind stops, the bullet still retains that sideways speed; it just doesn't increase. So the poi shift at 300yd will still be more than 200yd. Certainly it is would be more if the wind is there. Halfway is probably a good enough estimate.

In the example, the wind deflection up to 200yd is 0.5moa / 100yd. Should the wind be there from 200yd to 300yd, the poi would be off by 3*0.5*3=4.5". Without the wind, it would halfway between 2" and 4.5", or 3.25".

-TL

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[stagpanther]

I'll take a stab at it--per the previous discussion of object entrained in an air mass (of which it appears that I'm one of only two people who ascribe to this theory on this thread )--I do not believe this is correct--the object does not "aquire" a sideways velocity vector upon exiting an air mass.

 

[tangolima]

I'll take a stab at it--per the previous discussion of object entrained in an air mass (of which it appears that I'm one of only two people who ascribe to this theory on this thread )--I do not believe this is correct--the object does not "aquire" a sideways velocity vector upon exiting an air mass.

The bullet has sideways speed before crossing the 200yd line, correct? It cannot go to zero speed immediately unless a force of infinite magnitude is applied to it in the opposite direction.

-TL

PS. Notice that wind deflection is characterized as x moa / 100yd, instead of x inches / 100yd. That means the bullet is accelerating sideways in cross wind.

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[stagpanther]

The bullet has sideways speed before crossing the 200yd line, correct? It cannot go to zero speed immediately unless a force of infinite magnitude is applied to it in the opposite direction.

It does not--the air mass it is entrained in has a sideways movement relative to the fixed objects on the ground.

 

[tangolima]

It does not--the air mass it is entrained in has a sideways movement relative to the fixed objects on the ground.

Sorry I meant the bullet's speed relative to ground, to which the target belong.

Say the cross wind up to 200yd is 10mph. If the bullet is indeed moving with the air mass, its sideways ground speed is also 10mph. When it cross the 200yd line, it's ground speed cannot drop instantly to 0mph, unless some infinite "brake force" is applied.

-TL

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