These 20 mm cannons in long fairings are a straight replacement for the plastic items in your kit. They are made from turned metal and are very nicely done and well detailed. They are a definite improvement over what comes with the kit.
There’s a one page instruction sheet with an image of where to fix them and they should be easy to fit. You need to drill a 2 mm hole, then fix in place with cyanoacrylate, making sure the tip of the barrel stands 12 mm proud of the wing leading edge.
A highly recommended and simple update to your 1/72 Firefly build.
Many thanks to Master Model for the review sample.