This feller made himself some Gyrojet rounds. Their performance is… meh. I can see some clear ways to improve the design; lead weight in the nose of the case, nozzles swaged into the rear of the case rather than pinned, properly formed propellant, cast with a machined teflon mandrel to create a multi-fin grain for high initial thrust and very brief burn time. And use better propellant; the stuff he used is sugar-based… fine for amateur model rockets, but you want *good* propellant for this application. Ammonium perchlorate oxidizer, aluminum powder fuel, HTPB “rubber” as the binder. Throw in some iron oxide to goose up the burn rate, some superglue to speed cure time, and hey presto, about as good as it gets. Deployable fins might improve stability at the cost of additional cost and complexity.
Further: consider cutting down the case length and bonding a pointier brass, bronze, copper or steel nose to it to improve stability, aerodynamics and penetration. Other propellant grain options exist; one I’d like to see more work on is a thin sheet of propellant, perforated to turn it into a mesh screen, rolled into a tube with the layers separated by thin strips of bonded-on propellant. A whole lot of surface area, very fast burn, labor intensive to manufacture.