The gun barrels should be attached before installing the turrets because shortly after this step the decks are installed making it impossible to retrieve a turret should it get pushed into the hull. I should add that the turret fit is very tight and this isn't likely to actually happen but why risk it. There are two slightly different PE parts for the gun shields. Notice the half round opening on the side of the shield. Some allow the gun barrel to face forward and some aft. The final two pages of the instructions have some helpful photos of the completed ship and that's where I noticed the aft two guns face the rear and the forward three face forward. I used a piece of sprue to bend the shield and they fit perfectly. Be careful of PE parts A34 and 35 (step 3), the main deck stairways. I had to slightly re-bend one where the steps connect to the platform and it snapped in two. This presented no real problem because they can be installed in two pieces but the parts seem very thin and maybe are extra fragile. Some of the parts, should they break, won't be so easy to fix.
On a separate note, I had to ask Squadron for a replacement metal gun barrel because one of the main guns was cast wrong. In less than two weeks they not only sent me the correct part but the bag containing all of the gun barrels. If anyone needs a barrel let me know because I have a complete set minus one and two thumbs up to Squadron who clearly understand customer service.