Before I attempted to one of my own builds, I decided to have a go at the N7 Valkyrie build by Volpin Props: http://www.volpinprops.com/n7-rifle-mass-effect-3-2/.
My second mdf build, this one involved some sculpting and lots of sanding.
Glued and ready to stick
Body and stock
Body and bottom barrel
Top barrel done
Need to work on my weathering.
The build time was around 60 hours, over 2 weeks.