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Discussion Starter #1
So...I have a little itch to scratch for a small side project/hobby idea that I've been toying with, but would like some input.

Specifically I was wondering if anyone here had a general ballpark range of the G-Forces measured say either at the handguard or flattop of an AR/M4gery running either 5.56 or .223? The reason I'm asking is that my little toy project is going to require an accelerometer, which I have, but it maxes out at ~2000g's before it fubars itself according to the manufacturers specs.

I've read some unconfirmed reports of bolt actions like the .300's pushing 4000g's which I'm hoping is considerably more than something like an AR/M4. Basically I'm just trying to find out if the standard AR/M4 platform will register below 2000g's or if I need to go with something other than my little accelerometer chip that I'm not really in the mood to destroy.
 

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A round-counter?


-B
 

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Some things to consider:

Newton's 3nd law of motion: for every action, there is an equal and opposite reaction. So the gunpowder pushes the bullet forward and it pushes the rifle back (recoil). Because the bullet is light, it goes fast. Because the rifle is heavy, it goes slowly.

Measuring G forces is acceleration. 9.8 meter/second squared is one G. Gravity is also 1 G

Because it is semi-auto, there is a heavy bolt and buffer spring assembly that spreads out the rearward movement of the firearm out over a longer time. This changes things a little.

Because there is a lot of stuff clanging around in there the acceleration will be messy. The bolt starts locked, unlocks, moves rearward, and slams into the rear. This smacking around will have a very high acceleration. This is what you must worry about.

The point of maximum acceleration probably occurs when the bolt slams into the rear of the buffer tube. This is hard to calculate or predict.

Note when the bolt moves forward and loads the next round, there will be acceleration the opposite way.

On a bolt action, there are no internal moving buffers so all the rearward acceleration happens while the bullet is traveling down the barrel. Once it leaves the barrel the acceleration stops.

A way to lower the AR's smack in the bolt/buffer tube would be to put a Urethane disc in the back of the buffer tube. (It might effect reliability).

With your weight behind the rifle, the mass is a lot more then the rifle itself just (7 pounds + 160 pounds). Remember F = MA. The more mass you have attached to the rifle, the smaller the acceleration. This means you can double the weight of the rifle (by attaching on weights) and the acceleration will be halved. If you had the biggest guy you can find shoot it, the acceleration of the rifle will be less then a child...

One last thing: your 2,000 G sensor should be fine as long as it measures quickly enough. This whole thing happens really fast (a few milliseconds) so it needs to be quick.
 

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Since I have a few minutes, we can calculate an estimate. There might be some errors here as it has been a few years:

For a 5.56 round:

55 grain (.00356 kilogram) bullet starts from rest and leaves the muzzle at 5,250 feet/second (1600 meter/second). A 16 inch barrel (.406 meters) long. We have a 7 pound rifle (3.7 kilogram)

the equation we need is:
Velocity squared = 2 * acceleration * change in position

1600*1600 = 2 * a * .403

so the acceleration of the bullet is:

a = 3,176,178 meter/second squared or 324,100 G

Since you want the acceleration for the rifle, we know every action has an equal and opposite reaction:

The force on bullet is equal and opposite to the force on rifle

F = ma
F = .00346 * 3,176,178
F= 10,989 newtons of force on the bullet

and the opposite for the rifle:

F = ma
10,989 = 3.7 * a
a= 3,466 meter/second squared or 354 G

There you go... an estimate 350 G

Note this is an average acceleration. The max acceleration will be more (maybe double?). The clanging of the bolt will be much more. If you put a heavy person behind the rifle, it will be less...
 

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Ok Nutz...gotta ask what do you do for a living?
 

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PRACTICE and you'll never need to worry about accelerometer chips.....
 

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Discussion Starter #7
Wow...I didn't expect that much info back. Thanks Nutz... I'm still playing with some basics right now so I've got a long way to go, but that little explanation of yours brings me right back to grade school physics all over again.
 

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Ok Nutz...gotta ask what do you do for a living?
Absolutely nothing to do with this. Knowledge is power. :image035:

packinnova said:
I'm still playing with some basics right now so I've got a long way to go
Let us know what you are planning! Always do the math before you build. It got us to the moon...

Industrial Supply Equipment from MSC Industrial Supply look at "Big Book" page 1787, at the bottom, for the kind of urethane I was talking about. It is great stuff...
 

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Discussion Starter #9
Well, here is what I was planning... :rolleyes:

youtube video from this afternoon...
packinnova's AR Round Counter WORK IN PROGRESS

The initial logic development was done on an Arduino Duemilanove(atmega328 chip). I'm reading the inputs from a Freescale MMA7260Q accelerometer and dumping the parsed/edited results out to a 16x2 amber on black LCD.

Right now my code has the threshold for the y-axis of the accelerometer set really low so I can trigger it by hand for testing.

I still have a long way to go, but for all of 2-3 days worth of work I think I'm doing pretty good. I have a battery and charging system to add to it and then I can more closely finalize the wiring and mount this beast in an enclosure of some sort. Not a bad start though...

This whole project is more for my own fun on the side than anything else though so I'm having fun with it.
 

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Discussion Starter #10
This can also easily be modified to do a total round count for rounds fired throughout the day or week or whatever. I suppose it could be useful for folks trying to keep track of their rounds.
 

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So...I have a little itch to scratch for a small side project/hobby idea that I've been toying with, but would like some input.

Specifically I was wondering if anyone here had a general ballpark range of the G-Forces measured say either at the handguard or flattop of an AR/M4gery running either 5.56 or .223? The reason I'm asking is that my little toy project is going to require an accelerometer, which I have, but it maxes out at ~2000g's before it fubars itself according to the manufacturers specs.

I've read some unconfirmed reports of bolt actions like the .300's pushing 4000g's which I'm hoping is considerably more than something like an AR/M4. Basically I'm just trying to find out if the standard AR/M4 platform will register below 2000g's or if I need to go with something other than my little accelerometer chip that I'm not really in the mood to destroy.
As a career aerospace vibration test guy... my guess is, you won't destroy your accelerometer by subjecting it to accelerations higher then its rated measuring range, but - it depends on the accelerometer. Assuming you have a piezoelectric type, such as a PCB, Endevco, B+K, Kistler, etc - just look up what the accel's specs are on the mfr's web site. The "overload limit" is the parameter you're looking for, and that's typically 2-10 times the measurement range. As long as you're under that limit, you'll just saturate the output if you measure an acceleration higher than the rated range.

If you let me know what your accel is, I'll be happy to look up the specs and give you idea of its capability.

BTW, if you were setting out to buy an accelerometer for this measurement, you'd be better off getting a shock-type rather than a vibration type.

While some posts are quick to quote Newton's law, what is left out of the equation is the duration of the initial shock pulse. Hard to predict the acceleration on the receiver as a reaction to accelerating a 55-gr bullet to 3000+ ft/sec inside a 16-inch barrel.
 

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Since I have a few minutes, we can calculate an estimate. There might be some errors here as it has been a few years:

For a 5.56 round:

55 grain (.00356 kilogram) bullet starts from rest and leaves the muzzle at 5,250 feet/second (1600 meter/second). A 16 inch barrel (.406 meters) long. We have a 7 pound rifle (3.7 kilogram)

the equation we need is:
Velocity squared = 2 * acceleration * change in position

1600*1600 = 2 * a * .403

so the acceleration of the bullet is:

a = 3,176,178 meter/second squared or 324,100 G

Since you want the acceleration for the rifle, we know every action has an equal and opposite reaction:

The force on bullet is equal and opposite to the force on rifle

F = ma
F = .00346 * 3,176,178
F= 10,989 newtons of force on the bullet

and the opposite for the rifle:

F = ma
10,989 = 3.7 * a
a= 3,466 meter/second squared or 354 G

There you go... an estimate 350 G

Note this is an average acceleration. The max acceleration will be more (maybe double?). The clanging of the bolt will be much more. If you put a heavy person behind the rifle, it will be less...

Nutz... I did the same v^2 = 2as calc, but... try 3250 ft/sec as a typical 5.56 muzzle velocity, not 5250 ft/sec. That reduces the calculated acceleration by about a factor of 3 (I get about 123,000 g).

However, the measured acceleration will also depend on the relative rigidity of whatever is holding the rifle when it fires. If it's solidly mounted to a concrete wall, the measured acceleration will be vastly higher than if the gun was reacted against a block of foam rubber when it was fired.
 

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Nutz... I did the same v^2 = 2as calc, but... try 3250 ft/sec as a typical 5.56 muzzle velocity, not 5250 ft/sec. That reduces the calculated acceleration by about a factor of 3 (I get about 123,000 g).

However, the measured acceleration will also depend on the relative rigidity of whatever is holding the rifle when it fires. If it's solidly mounted to a concrete wall, the measured acceleration will be vastly higher than if the gun was reacted against a block of foam rubber when it was fired.
I thought 5,250 FPS was a little on the High side. Some of the super flat shooting calibers including the Chey Tac and the .338 Extreme Tactical are a lot slower than that.
http://www.xtrememachining.biz/338tactical.html
 

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Discussion Starter #14
I added the total round count...not sure if it will stay or not, but it's there now...

If I have time this weekend I'm going to try to build in the connections for a LiPo battery pack. Once I'm sure everything works on the battery pack then i can start trying to figure out how to build it into some sort of enclosure for mounting and more testing.

I'm wondering if I should split up the display into columns so it's less chance of being confusing like maybe
LineOne: Remaining | Fired
LineTwo: 10 | 20

Any thoughts?

 

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Discussion Starter #15
well...I'm still playing so I added a bit more functionality. It now has not only rounds remaining, and rounds total fired, but now a momentary pushbutton type switch for resetting the total rounds fired count...
boobtube link...
pushbutton reset
 

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Discussion Starter #17
Yeah at the time the idea was just something I would be interested in but force me to learn something new. It's just another potential shiny gadget to hang on the end and weigh down an otherwise perfectly good AR.:rolleyes:
 

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Sorry I haven't been paying attention. I don't recall where I pulled the 1,600 m/s I used. I suppose it is high. :)

Gasmitty, sounds like you do some neat work! I have a friend who does aircraft vibration testing and flaw detection with some sort of ultrasonic equipment. He works for Northrop Grumman...
 

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However, the measured acceleration will also depend on the relative rigidity of whatever is holding the rifle when it fires. If it's solidly mounted to a concrete wall, the measured acceleration will be vastly higher than if the gun was reacted against a block of foam rubber when it was fired.
I don't quite follow. With a given force and impulse (duration of that force), don't heaver things accelerate more slowly?

A rifle mounted rigidly to a concrete wall would make the total mass very high so the average rearward acceleration would be small...

I am not too knowledgeable about vibrating stuff or this high force/short impulse. Are you explaining that a rigidly held rifle will vibrate more violently then a padded one? By vibrating violently, parts of it will have high instantaneous acceleration?

Is vibration/metal flexibility more of a problem in a rifle or the average reward recoil from driving the bullet (a la conservation of momentum) ?

thanks in advance!
 

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I don't quite follow. With a given force and impulse (duration of that force), don't heaver things accelerate more slowly?

A rifle mounted rigidly to a concrete wall would make the total mass very high so the average rearward acceleration would be small...

I am not too knowledgeable about vibrating stuff or this high force/short impulse. Are you explaining that a rigidly held rifle will vibrate more violently then a padded one? By vibrating violently, parts of it will have high instantaneous acceleration?

Is vibration/metal flexibility more of a problem in a rifle or the average reward recoil from driving the bullet (a la conservation of momentum) ?

thanks in advance!
Happy to respond... I recognize an open mind when I see one. Pardon me if I stray from a purely academic explanation.

First off, I think I introduced some error by mixing up mass with stiffness. Heavier and stiffer are two different things.

Consider the case of a cannon ball being dropped from a given height onto first, a thick steel plate, and second, onto a big rubber ball. In each case, the force of the cannon ball imparted onto the target is the same - F = ma, the mass of the cannon ball times the acceleration due to gravity. Let's say the steel plate weighs the same as the rubber ball. But what differs in each case is the duration of the impact event. If you had a load cell under each target, the cannon ball hitting the steel plate would have a very high measured force, but with a very small time duration. Conversely, the impact on the big rubber ball would have a lower peak force but the impact event would take place over a much longer time period. In each case, the impulse (F*dt, or integral of force over time) would be nearly the same.

Now let's interject that momentum is the product of mass and velocity, and it bears the same units as impulse, the product of force and time.

Back to my misguidance - since the mass of the targets in my cannonball example are the same (by definition), and the force of impact is the same, what varies between the two cases is acceleration and time. To arrive at the same momentum, if one has a longer duration (the 'soft' target, the rubber ball) is must have a lower acceleration, and likewise, the steel plate will have a shorter duration and a much higher acceleration.

Are you explaining that a rigidly held rifle will vibrate more violently then a padded one? By vibrating violently, parts of it will have high instantaneous acceleration?

Yes. What is different is that the duration of the equal and opposite reaction will differ, depending on the elasticity (stiffness) of the system against which the bullet's firing is being reacted. High stiffness, short duration; low stiffness, long duration.

Is vibration/metal flexibility more of a problem in a rifle or the average reward recoil from driving the bullet (a la conservation of momentum) ?

Momentum is of course being conserved. m1V1 = m2 V2, so if m2 is a lot bigger than m1, V2 will be a lot lower than V1. But what needs to enter into the equation is the duration of each event; in the initial event, the firing of the bullet takes place over a very short period and imparts a huge acceleration to a small mass. The reaction takes place against a larger mass, and the duration of the reaction depends on the stiffness of the system being reacted against.

An even simpler model would be to consider a rifle hung by two threads. The gun fires, and the gun recoils over a certain period of time and it swings backward a certain distance. Double the weight of the rifle, and the distance it recoils will be less than for the unweighted gun.

Now take the same model, but this time there is a coil spring between the butt of the gun and a solid wall. When the gun fires, the recoil is reacted by the combined stiffness of the spring and the solid wall. Now vary the stiffness of that coil spring... the mass hasn't changed, the recoil force and momentum haven't changed... but the acceleration and the velocity have changed.

Does that make sense, or have I added more confusion?
 
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