I just grabbed up a variety of modern nominally 2 3/4 inch fired plastic hulls from my supplies. Winchester compression formed AA 12- and 28-gauge and a 12-gauge Super-X Pheasant, Remington Premier STS 12- and 28-gauge and 12-gauge Remington Express, and a 20-gauge Active all measure between 2 5/8 and 2 11/16 long. The only hull I picked that came near 2 3/4 inch was a Western Xpert Mark 5 16-gauge and it was still abour 3/64 shy. A 2 1/2 inch compression formed AA .410-bore shell was an honest 2 1/2 inch.
Just grabbed some fired old paper cases that have been sitting on my bookcase. The 20-gauge Peters High Velocity, the 20-gauge Remington Nitro Express, the 20-gauge Federal and the 28-gauge Western Super-X are all 2 21/32" long.
I measured all these shells sitting upright on a flat surface with a machinist's scale.
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