Sorry you lost your rocket, bummer.
A couple thoughts to add:
(1). Our first L1 cert flight had the same profile, just lower altitude and easier to see what happened. A little masking tape on the coupler tube jammed in the cold weather, rocket comes in ballistic, main charge goes off shredding the harness and sends the parachute off on it's own.
(2) if you recovered your altimeter you should be able to see the pressure spikes from the failed deployment charge and the motor ejection. There should be two spikes-a smaller one for the deployment charge and a much bigger one from the motor eject-we had a motor eject not blow once but that doesn't sound like the case here. It would also help rule out an altimeter glitch.
(3) +1 on the VMAX delay grains. We've flown three so far and none of them blew at the right time. One lawn dart (with ejection while the rocket is sticking out of the ground) and two hair raising deployments. Others may have had better luck but I don't trust them.
(4) Last thought: you didn't mention it but do you use redundant electronics? It adds $50 to the costly the rocket, but can be a rocket saver. We always set our backup charges to be 150% of the main charge or 0.5g, whichever is more. In this case it may have helped since the primary charge was on the small side. Buys a lot of peace of mind too.
Don
