Cardboard is easy to work with. You just have to use it right. Dont make the gun hollow. Make it solid to add weight to the weapon to make it more realistic. Its better than wood if you harden it before painting with Resin and round the weapon were needed with Bondo. If you need pictures of cardboard weapons, or have any questions about how to make them the best possible way, PM me and I will do my best to answer.
EDIT: I will post a tutorial on how to make a cardboard weapon as soon as I have good cardboard. It will be on how to make the sniper rifle seeing how that is my current project other than my battle rifle.
Yea, keep everything you want to be removeable, moveable, or lighten by a LED hollow. Mag well, keep it hollow. Safety button if you want it moveable, make sure that you have a hole with a stable wire or something in the middle of it. Add the safety button afterwards. Charging handle, if you want it to be abled to go back and sprint forward with a sprint, hollow out the area for the handle and cover piece. It takes a lot of work, but every function plus a working trigger are in my BR, so its highly possible to do the same. Just make sure that the BR is full scale with no mistakes or anything. Once you make one mistake when you scale the blueprint up, your screwed if you do not correct it.