This Master Model set provides10 barrels: Two machined aluminum 15”/42 Mark I barrels, eight machined brass 4”/45 Mark XVI barrels, and a single page instruction sheet.
You will need to cut the styrene barrel off and drill a hole in the plastic breach. The directions call out a 1.5 mm bit (~#53) for the 15”/42 Mark I barrels and a 0.5mm bit (~#76) for the 4”/45 Mark XVI barrels. Be careful to center the drill bit on both x-axis and y-axis.
Be sure to use your favorite CA (super glue) or epoxy, as the normal plastic glues or solvents will not react with the aluminum or brass. Be careful painting the barrels, especially the brass ones since paint can easily foul the hollowed out muzzles.
Of note is the re-sealable packaging that Master Model uses making the parts easy to review and stuff back into the package securely. That being said, you will want to be careful handling the brass barrels as their small makes them quite easy to disappear.
As always, check your reference photos for placement and alignment.