I'm sure this is the tutorial you've all been waiting for... the big freaking gun. Now, after building this I have ideas on how to improve upon it. So, I'll add a new tutorial once I finish the upgrades detailing how I did those. Hopefully, this tutorial will point you in the right direction.
First, you need a scale outline of the gun. I'm not going to go over how to make a scale outline here. There are numerous tutorials online, and I may make a video for this at a later date. I also like to sketch my patterns out on paper to get an idea for the design. It helps me, but it's not necessary.
Once you have your scale outline, print and cut it out. Trace on to foam and cut that out. Be sure to use a sharp blade, and to continuously sharpen your blade while cutting. I used 10mm foam from tnt cosplay supply, and cut out 4 of my outline.
Get a dowel rod in the diameter you like for the barrel. I used 7/8, i believe. Measure where it should sit, mark, and cut out of 2 outlines.
Glue together using contact cement, encasing dowel rod. Do this carefully, trying to keep everything even as it will save you from loads of sanding later.
Sand. I used an orbital sander, dremel, and hand sanding. I did not get everything smooth due to time. Therefore, I used copious amounts of kwik seal to smooth things out as best as I could. I also did not sand in details (ie, the stock). I will add that to my update tutorial once I do it. Once sanded, you can begin adding details. I used the same process I used for my outline to trace out some details. Transfer to your gun base, cut with knife, and heat with heat gun. Be sure to use a low setting and to keep the heat gun moving so you don't burn your foam (like I did).