Master Model has released three new gun barrel sets for the P-38 Lightning, one of my favorite aircraft. This one is listed as the early version with 4 x .50 cal. Brownings, with drilled cooling jackets & early and late 20mm cannon tips. These are mostly seen on early versions of the plane but the instructions have photos of some later birds sporting the drilled cooling jackets, so check your references.
The detail is beautiful with drilled out jackets and barrel ends. The early 20mm also has some additional detail.
Construction depends on which kit you are using. Many have separate barrels so these are just simple replacements. Other kits (like the Hobby Boss) will need to have the original barrels cut off and holes drilled for the barrels. Either way the detail on these is much better than any of the kit guns.