The other day at the range I managed to get a good video capture of the bolt carrier cycling on my AR-10.   The engineer in me couldn't let good kinetics data go to waste so used the frame captures to calculate the bolt location during cycling.  In case you ever wondered about such things here are the velocity, acceleration, and position profiles for the bolt group in an AR-10 firing M-80 ball at NATO velocities.

Specs are: 16" Armalite chrome lined barrel, rifle length gas system, standard Armalite bolt group, shortened lightweight AR-15 CAR buffer, AR-15 CAR buffer tube and shortened CAR spring.

video capture rate was 60 frames per second:




anyway, I thought it was interesting.  especially interesting to me was that the bolt carrier only travels about 0.011" by the time the bullet leaves the muzzle, and this is assuming constant acceleration of the projectile in the bore.  in reality, the bolt carrier probably moves about half that in that time.  pretty cool if you ask me.

My life is now complete.








Seriously though, cool stuff, and thanks.

That's all well and good, but what's the airspeed velocity of an unladen swallow?

not sure, but I did hit a sparrow the other day on the highway going about 70.  he was probably doing half that when he met my grille.  poor bastard.

Can you try it on a treadmill?

That's pretty cool. Thank you.

Must be awful fast to bend your rifle in half like that

African or European?

this rifle shoots so fast it warps time and space with each shot.  the barrel is actually still straight, it just has the illusion of curvature due to the warped space-time

Damn, you need to start handloading some viagra 308, because your rifle is going limp!

12.83 fps is only 9 mph.   Seems kind of slow.

it does seem slow, doesn't it, but it is correct.  the distances involved are so short and the accelerations are so abrupt that even with such low peak velocities it still completes a whole cycle in 0.083 seconds (723 rpm).

The initial velocity of the carrier (before bolt unlocking) is more in the 30 feet per second range.  Look at the stroke length of the gas system, dictated by the vent holes on the carrier.  IDK what this is in the AR10, in the AR15 it is 0.215".  The peak velocity happens when the carrier has moved this distance.  From there on, the spring is slowing the carrier+buffer down.  



I've worked this system on the mass-energy relationship to come up with these figures.  You can safely assume the spring energy over the 0.215" distance to be insignificant compared to the 1/2mV² of the carrier+buffer+half spring mass.
 

at no point in the cycle could the bolt carrier be moving at 30 fps, at least in the video I captured.  the spring rate is nowhere near enough to slow the bolt group from 30 fps to 12 in less than an inch, and if it was the bolt group would only cycle about an inch, total.

perhaps you are confusing acceleration with velocity?  the initial acceleration numbers are extremely high, but the peak velocity is never above 15 fps, period, the physics just arent there.

Maximum bolt+carrier+buffer velocity happens after the first 0.215" of movement, meaning there is about 2.25" of travel for the spring to absorb the energy of this combined mass.  I have no doubts of the remaining 1.25" of travel would cause the velocity to drop to near zero.  In the AR15, port pressure is 15ksi.  The net area of the bolt acting upon by this pressure is about 0.147 square inches.  Meaning the force on the carrier is over a ton of force.  2200 pounds.  Force is equal to mass*acceleration...do the math.



 

I have done the math, port pressure does not equal the pressure at the carrier due to pressure loss in the tube, plus the pressure is highly transient so attempting to make guesses as to what it is in the carrier and when is an exercise in futility without measuring it in real time.  It is NOT being acted on by an ideal gas at constant pressure, even at the gas port, much less at the carrier itself, which is what you are assuming in making your calculations.  Gas undergoing chemical reaction, at transient temperatures, and transient pressures, passing down a high aspect ratio tube at high velocities is about as far from ideal gas conditions as possible, any assumptions made will be orders of magnitude off.

The fact of the matter is that I actually measured the bolt velocity using real time imaging and the initial peak carrier velocity is 12.8 ft/s, the carrier slows to 9 ft/s before it impacts the rear of the buffer tube, and the whole assembly returns after bouncing off the buffer wall with an initial velocity of ~5 ft/s and accelerating to ~9 ft/s by the end of its travel.

If you sprung the carrier to have zero velocity when it impacted the rear of the buffer tube, it would only work for one specific load and you would have very little room for changes in port pressure.  You would be constantly on the verge of short stroking, at least in a rifle like the AR.  Some rifles with longer bolt travels and heavier bolts DO reach zero velocity before they hit the travel stop, I have seen it in high speed video, but the AR-10 is not one of them.

This thread rocks

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at what point does the rifle itself start moving?  At firing or when the bcg starts moving?  and does that affect the measurements?

 I don't know that!

it moves as soon as the projectile starts moving in the bore, but the mass of the rifle is so large compared to the projectile that its velocity is tiny compared to everything else.

If only my Engineering Physics I class used guns in thier examples.  I might have paid a lot more attention.  Instead we just filmed sleds moving up and down an air track and did the same calculations as above.  Currently studying Gauss's Law, electric flux, capacitance, and a few other fun topics in preparation for my Physics II test.

Spiff- Thanks for that. I read very recently that HK designs to max a bolt velocity of IIRC 4 Meters/sec which is about 12 FPS.

Fantastic thread. Really, really cool.

I saw some brass from SCAR rifle testing a couple of years ago (NSW....OPSEC...) that had a slight buldge in the transition from the neck to the shoulder. 7.62 and 5.56. I would assume that the brass was starting to eject while there was still a considerable amount of pressure.

That's really useless information, but awesome information.

Ah, very cool, if I was running a normal weight buffer instead of the lightweight buffer in this rifle it would probably be a lot closer to 12 ft/s max velocity, on the money.

It does seem to be a good number, it provides plenty of time for the magazine to feed the next round and would give a very reasonable cyclic rate in full auto, around 680 rds/min, while providing plenty of momentum to cycle the gun in adverse conditions.

I think that I am going to have to test some more rifles to get more bolt velocity data, I like the theory that ~12 ft/s is a good bolt velocity number and I am curious as to how many rifles follow that pattern.

yea, probably, or the chamber was cut funny.

When a fly lands on the ceiling does he invert at the last moment or does he fly an inside loop?

Fair enough but when you work out the spring rate of the buffer spring, then back calculate the velocity of the carrier+buffer required to fully stroke the carrier, you will find it takes at least 17 feet per second of initial velocity.  Because this discounts the hammer spring force and its mass along with friction, initial velocity of the carrier must be substantially higher than 14 feet per second.



On the AR 15 rifle, the buffer spring rate is about 1.6 pounds per inch by my measurement.  Since the spring is preloaded by 5" or so, that makes initial buffer spring force to be 8 pounds.  With a 3.75" carrier stroke, that makes the final buffer force to be 14 pounds.  Carrier and buffer are about a pound.  That makes the spring energy to be 4-11/16 foot pounds.  Energy based on carrier velocity is 1/2*m*V², solving for V you get 17.37 feet per second



Clearly, the port pressure losses are minimal.  And the hammer cocking forces with the hammer inertia are more than insignificant.  Friction of the carrier is however, minimal since there are very little lateral forces.



 

What's the best kind of popcorn for cool nerd debates?

That's it.  I'm calling it.  You are Watson. They stuffed you inside a computer shell and put you on Jeopardy didn't they?



 

just finished that last semester. do you cover optics in your physics 2?

FWIW- mine did, and electrical circuits.  My physics professor did use guns and bullets frequently for examples- it was still too dry to be interesting.



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