The gun troughs sit at the top of the engine accessory bay, although they are largely hidden beneath the uppermost cowling. They form the lower half of the channel through which the forward-firing .50" machine gun barrels pass. While the gun troughs may be the smallest sections of engine accessory bay cowling, they are perhaps the most complex. Their multitude of compound curves make them a challenge to manufacture. The project arrived with a set of these cowls, but they had to overcome significant issues on the road to airworthiness.
The Lefthand Unit:
The lefthand gun trough suffered from corrosion, dents and cracks - not to mention a hasty wartime repair to its aft end (see right side in below image). Pioneer's engineers exerted considerable effort to address these issues. They replaced the wartime repair with a structurally correct example, and spliced in a replacement section of the aft rib to account for a crack in one of the mounting holes.
The Righthand Unit:
The righthand gun trough was in an even more distressed condition than the lefthand unit. It suffered from corrosion, dents, and cracks along with a partially crushed section as well. Pioneer’s technicians repaired all of this damage but had to replace the aft rib, which was cracked beyond use.
Pioneer's technicians trial-fitted the two gun troughs into the engine accessory bay cowling assembly to verify that they would connect correctly. Given the level of preparation prior to this moment, it should come as no surprise that everything fitted exactly as it was supposed to.