ok hear me out

The demon core's shell isn't shielding, it's reflective and is what causes the fission material to go supercritical. The only shielding on this gun is the lead barrel. Once the two halves are closed, the entire sphere would go supercritical. Importantly, this wouldn't cause an explosion but would instead bathe everything around it in fatal levels of radiation as the sphere rapidly heats up and starts melting. Since the lead barrel is the only shielding, anything directly behind the barrel would receive less (though probably still a fatal amount) of radiation.

Because of the circular hardened barrel pointed at the target, it would be the least likely direction for shrapnel to be thrown when the whole thing fuckin explodes

It would explode generally, and disperse bits of the core all about. Congrats! You have a dirty bomb, and all the physicists in other countries will laugh at you. (See the DPRK tests in the aughts)

This is why we invented the fat man configuration, which then powers the Teller-Ulam configuration in most hydrogen bombs.

Remember that the demon core and reflectors were lab bits used for the development of the atomic bomb configurations. Little Boy was named in the convention of the Tallboy earthquake bomb, and was a gun design that shot the core with enough force to prevent dispersal before complete fission, and Fat Man evenly compressed the core in all directions, again with enough force to prevent dispersal before complete fission.

I think they are implying that when it is nearly fully enclosed it would go super critical...and kaboom.