I did some quick edits to the shotgun model so that no pieces will be larger that one page. Of course, if anyone actually wants to use this model, they are going to have to re-unfold the model.
Or you can just cut the pieces on each page out separately and on one of the pieces where one side meets a page make it a tab and just glue it together. it will act like a whole piece.
well laddie, the numbers are pretty easy to put on. first you get a sharpie marker. next, you markup every part... jk. anyway, how you put the numberings on the part is relatively easy. first, if you have the model open up in front of you, open up the bar titled "2DPatternWindow". when you open that up, move down the list until you see a button marked "Show Edge ID". when you click this, you should see the numbers, if you zoom in close.
It has been my experience that one should never rely entirely on Pepakura for making weapons and armor. Honestly, there are just some things that are more difficult to make with paper than they are to make from scratch (narrow, circular, and long pieces expecially.) The barrel is one example. Find some other object that you can use in place of it, like a length of PVP pipe of approximately the right diameter and cut into size. Then place the rest of the Pepakura around that as you go, using that as a substitute for the paper barrel. It will probably save you a lot of frustration in printing and assembling it, and you will want to use something rigid for the barrel anyway.