I recommend you simplify the model until you get something running. I cut off the second half of the barrel.
In DesignModeler, I used a plane to slice the bottom of the barrel off, then I put that and the barrel into a multibody part by picking them and selecting Form New Part. I assigned Multizone mesh control to the bottom of the barrel. This allows the mesher to make a nice clean mesh on the barrel and connects them using shared topology.
I also cut the tip off the projectile to get a nice mesh. You don't need a pointy tip since there is no air in your simulation.
Note in the Mesh Details that Size Function is Proximity and Curvature and Num Cells Across Gap is set to 2.
The Projectile and Charge had a Sizing mesh control on the bodies of 15 mm.
Don't forget to suppress all the automatically generated Bonded Contacts!
I haven't done the research to know how detonation and explosives work in Explicit. I hope you have.
I did this in 19.0 so you can't open my file. Do these same few things to your file and send that back to me.
Are you on the limited Student license or an unlimited Research license?