Via email from Rolf.
.. is made by necking down a 20mm Vulcan autocannon shell casing. It was developed as a collaborative project among members of the Snipers Hide online forum. It has undergone several revisions since 2010.
It’s usually loaded with a hand-crafted low-drag 1,690 grain solid copper bullet, which it fires at roughly 3,350 fps. The projectile itself is roughly 4 inches long, with driving bands and an aggressive rebated boat tail to reduce base drag.
The sectional density is around 0.70, which is an astoundingly high measure of the mass-to-diameter ratio. It’s very heavy for it’s caliber, nearly a quarter pound.
The ballistic coefficient is about 1.85, which is an astoundingly high measure of how aerodynamic the bullet is during flight.
And, of course, because it is more that .50 caliber (for now) it must be registered as a “destructive device.”
Wow. Nice ballistics! But I wouldn’t want to pay the price to feed it.