Your neck thickness sounds fine. Mine are .289 also. Allow .001" clearance for safe release of the bullet and it is within the .300 SAAMI chamber blueprint for Grendel.
Your case length is also fine at 1.51", within spec.
Your expanded (fired) neck diameter is also fine at .299". Allowing for .001" springback it sounds like your neck has been reamed to Grendel SAAMI spec.
So, it is not the case size, length or chamber that caused the 200fps spike.
It is too large to be within the error parameters of a Magnetospeed. The Magnetospeed doesn't suffer the vagaries of clouds like the Chrony. Being the only one that was out 200fps means it's not the fault of the chronograph.
If it was a doughnut at the base of the neck it would either not go into battery or fire normally, but certainly not spike 200fps.
This leaves human error I am afraid; an overloaded case, a slip of the pen when transcribing. I've done this before more times than I care to admit...it happens.
I recommend full-length sizing ALL those second hand cases back to SAAMI, regardless if you are using a bolt-gun or auto-loader. Preferably all at the same time. Screw the die right in for a one-time only sizing back to Square One. Then, once they are fired and fire-formed to your chamber unscrew the die slightly if it is an AR and bump .003-.004" to prolong case life. If it is a bolt gun, swap to neck-sizing and bump .001" for accuracy and case life.
Your case length is also fine at 1.51", within spec.
Your expanded (fired) neck diameter is also fine at .299". Allowing for .001" springback it sounds like your neck has been reamed to Grendel SAAMI spec.
So, it is not the case size, length or chamber that caused the 200fps spike.
It is too large to be within the error parameters of a Magnetospeed. The Magnetospeed doesn't suffer the vagaries of clouds like the Chrony. Being the only one that was out 200fps means it's not the fault of the chronograph.
If it was a doughnut at the base of the neck it would either not go into battery or fire normally, but certainly not spike 200fps.
This leaves human error I am afraid; an overloaded case, a slip of the pen when transcribing. I've done this before more times than I care to admit...it happens.
I recommend full-length sizing ALL those second hand cases back to SAAMI, regardless if you are using a bolt-gun or auto-loader. Preferably all at the same time. Screw the die right in for a one-time only sizing back to Square One. Then, once they are fired and fire-formed to your chamber unscrew the die slightly if it is an AR and bump .003-.004" to prolong case life. If it is a bolt gun, swap to neck-sizing and bump .001" for accuracy and case life.