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.
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?