/give @p firework_rocket{Fireworks:{Explosions:[{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]},{Type:4,Colors:[I;15790320],FadeColors:[I;11250603]}],Flight:3}}

And rename it to: 12cal ; 762mm ; PG7VL

Take the firework in the second hand and reload the crossbow