Hmm, would the accelerometer really detect anything? I think the bullet would have to experience significant lift, otherwise it's in free fall and the only acceleration you'll see after it leaves the barrel is air resistance, which isn't in the right direction to change with each rotation. To a first approximation, spinning bullets don't experience lift.

Maybe there's still some minuscule effect that can be detected, though. The sections on Magnus force in http://www.nennstiel-ruprecht.de/bullfly/index.htm seem to suggest that this is the case. But now I'm not so sure it's as simple as counting the number of periods of oscillation from the accelerometer.

IANAIBOEP (I am not an expert in ballistics or even physics)

I was thinking of the accelerometer being located near the outside of the shell, rather than relatively stationary near the centre of rotation. I agree it's not entirely trivial, but it seems pretty doable to me.

The accelerometer cannot measure the earths gravity which was probably what the GP suggested. However you can measure the centrifugal force (http://xkcd.com/123/) which may be more precise than trying to measure the muzzle velocity or just assuming certain known muzzle velocity.

No. An accelerometer measures change in acceleration.

When the round is fired, it will experience a huge acceleration from the propellant along its axis plus a very large rotational acceleration as the rifling spins the round from 0 revolutions per second to a large number of revolutions per second.

Once the round leaves the barrel, it is falling at a constant acceleration (9.8m/s, freefall, 0G, "weightlessness"), so it will not be able to measure the earth's gravitational force. It also will be spinning at a constant rate because there is no outside force increasing or decreasing the spin rate, thus no rotational acceleration. While there will be acceleration due to the centripetal effect of the rotation, that acceleration is constant and thus not usable with an accelerometer for counting revolutions.

What? An accelerometer does measure acceleration, but relative to the free-fall acceleration.

That's different than measuring the change in acceleration.

(http://en.wikipedia.org/wiki/Accelerometer)

I got it wrong in my first sentence - in order for an accelerometer to be used, there has to be changes in acceleration proportional to distance travelled. Per the rest of the discussion, there isn't any.

You still have it wrong. An accelerometer measures acceleration.

Users usually calibrate the device at rest to get a good estimate of G so that it can be subtracted from accelerations measured later. Is that what you mean?

No. A falling object is at "0G", it is "weightless" (in a "drop seat" type carnival ride, when the ride triggers, you have no force on your butt). A bullet is a falling object after it leaves the gun. You cannot measure earth's gravitational attraction from the bullet's frame of reference because it is 0. Therefore, you cannot use earth's gravitational attraction to measure the bullet's spin rate because the value is zero, and zero has no direction information.

Sure, once the bullet is moving ballistically you can't measure G. My argument was with the statement that accelerometers measure changes in acceleration. They don't.

All the positive acceleration happens in a very short time. It might be hard to measure it frequently and accurately enough to use the integrated results as a velocity estimate.

Perhaps measuring the negative acceleration caused by air friction could give you a velocity estimate (the velocity-to-friction function is probably non-linear). But this may vary a bit with air density, temperature, etc.

I'd be interested to see how it's done. My bet is that it's not directly measuring a near-impulse acceleration.

> Surely it's just a 1 axis accelerometer and perhaps a lookup table or some basic maths?

I'm more interested by the mechanism that programs each projectile's detonation distance. That seems like the tricky bit...

Something along the lines of RFID? I've heard them described as "radio controlled" before too.

Since it's in the chamber protecting it from external sources of interference / attack (e.g. programmed to explode at the end of the muzzle) should be easy to protect against physically. I guess you could do it with some mechanically retractable pins or something, but electrical contacts and the inside of a bullet chamber don't seem like the sort of thing that should go together to me. You'd need to clean them constantly.

Presumably they got their security right; no recommit after firing, for instance.

If not, I can easily imagine a "bullet jammer" gizmo that broadcasts a retargeting command. "Sorry, little grenade, you have another thousand meters to go...."