It is a cart that holds a catch/containment device to stop a very fast moving projectile.
Although designed completely wrong, it's longevity has surprised everyone. It's actually undergone 2 revisions/modifications to continue it's use. The pictures below represent very low power shots as you can surmise from the catch can being used. That design was a temporary fix until the box I constructed from 2" armor plate was completed. They destroyed that, and I recently completed construction of a 6" thick armor plate barbette that's currently in use.
The twisting and collapsing you see is due to the force of the projectile being stopped/redirected. Were it me I'd have done it completely differently, but that's me.
