speaking of defensive rifles...

speaking of defensive rifles...

This is a discussion on speaking of defensive rifles... within the Defensive Rifles & Shotgun Discussion forums, part of the Related Topics category; 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 ...

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  1. #1
    VIP Member Array packinnova's Avatar
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    Question speaking of defensive rifles...

    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.
    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)


  2. #2
    BAC
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    A round-counter?


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  3. #3
    Distinguished Member Array nutz4utwo's Avatar
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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.
    "a reminder that no law can replace personal responsibility" - Bill Clinton 2010.

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    Distinguished Member Array nutz4utwo's Avatar
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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...
    "a reminder that no law can replace personal responsibility" - Bill Clinton 2010.

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    VIP Member Array jwhite75's Avatar
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    Ok Nutz...gotta ask what do you do for a living?
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    VIP Member Array edr9x23super's Avatar
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    PRACTICE and you'll never need to worry about accelerometer chips.....
    "Guard with jealous attention the public liberty. Suspect everyone who approaches that jewel. Unfortunately, nothing will preserve it but downright force. Whenever you give up that force, you are inevitably ruined". - Patrick Henry

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    VIP Member Array packinnova's Avatar
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    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.
    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)

  8. #8
    Distinguished Member Array nutz4utwo's Avatar
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    Quote Originally Posted by jwhite75 View Post
    Ok Nutz...gotta ask what do you do for a living?
    Absolutely nothing to do with this. Knowledge is power.

    Quote Originally Posted by packinnova
    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...
    "a reminder that no law can replace personal responsibility" - Bill Clinton 2010.

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    VIP Member Array packinnova's Avatar
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    Well, here is what I was planning...

    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.
    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)

  10. #10
    VIP Member Array packinnova's Avatar
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    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.
    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)

  11. #11
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    Quote Originally Posted by packinnova View Post
    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.
    Smitty
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    Quote Originally Posted by nutz4utwo View Post
    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.
    Smitty
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  13. #13
    VIP Member Array Rob99VMI04's Avatar
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    Quote Originally Posted by gasmitty View Post
    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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    VIP Member Array packinnova's Avatar
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    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?

    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)

  15. #15
    VIP Member Array packinnova's Avatar
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    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
    "My God David, We're a Civilized society."

    "Sure, As long as the machines are workin' and you can call 911. But you take those things away, you throw people in the dark, and you scare the crap out of them; no more rules...You'll see how primitive they can get."
    -The Mist (2007)

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