My boy Archie figured some stuff out a long time ago! Unfortunately he didn’t get the story straight about phlogiston

Bouyancy is based on how much water is displaced, not amount of air inside the container. 66x12x13=10,296 cubic inches. Fresh water density is about 0.0361 lb/in^3 so that case has to displace at least 372 lb to sink.

Metric would be better for this but we’re talking about a stinger missile here.