Lawrence,
The twin manual chambering does seem to coincide with a different MPI relative to all the other single manual chamberings or auto-loading. As you know, a bullet at the top of the stack sits above the forward wall of the mag so usual OAL limits do not apply. It could have moved forward twice and still able to be fed from the mag. Maybe a bullet sitting against your lands on start goes low, although the opposite was my experience.
If the bullet is moving on chambering the final OAL could also be shorter from being pushed back, twice as short for that particular round.
It's unsurprising that a new barrel displays higher pressures due to being a bit rough, or at least until a few firings have lapped it. The TBox may also be a tight barrel.
You list 30.5gns of water for the 6AR in your QL analysis. Is this correct for your cases? I don't shoot the 6AR but if you believe Wikepedia it's 36gns of case capacity, same as the Grendel 35-36gns. If it's a Grendel case necked down I wouldn't think there's a full 5gns difference. Certainly make a big difference in your pressure predictions.
The twin manual chambering does seem to coincide with a different MPI relative to all the other single manual chamberings or auto-loading. As you know, a bullet at the top of the stack sits above the forward wall of the mag so usual OAL limits do not apply. It could have moved forward twice and still able to be fed from the mag. Maybe a bullet sitting against your lands on start goes low, although the opposite was my experience.
If the bullet is moving on chambering the final OAL could also be shorter from being pushed back, twice as short for that particular round.
It's unsurprising that a new barrel displays higher pressures due to being a bit rough, or at least until a few firings have lapped it. The TBox may also be a tight barrel.
You list 30.5gns of water for the 6AR in your QL analysis. Is this correct for your cases? I don't shoot the 6AR but if you believe Wikepedia it's 36gns of case capacity, same as the Grendel 35-36gns. If it's a Grendel case necked down I wouldn't think there's a full 5gns difference. Certainly make a big difference in your pressure predictions.