Step 5: Barrel Assembly step 2
The 3" pipe's length is fairly subjective, I made mine around two feet, but I made that decision based on overall height. With the barrel assembly sitting flash suppressor side down on the floor I wanted the back of the entire thing to be at about armpit level which for me was 49". Depending on how high you want it to be relative to yourself you may want to shorten or lengthen the 3" pipe, although if you change it too much you may also want to change the lengths of the 1 1/4" barrels so it doesn't look out of proportion.
Now your barrel is, essentially, complete. You should cut pieces of EPS foam to fill the gaps between the barrels and the stabilizer rings like we already did for the flash suppressor, but you can do that later if you'd like or not at all if you don't mind the way it looks with the gaps.
A note on cutting the EPS foam: I used a sharp knife and a jigsaw, the real tool for doing it is a hot wire cutter but I don't have one and didn't want to buy/make one. If you have one that's great, working with the EPS foam will be a lot faster and easier, not to mention less messy. If you don't you can get by, you just need to be more careful with cutting it so you get clean-ish edges and realize it will take more time and be the biggest headache involved in the entire project.