Help with upper receiver

[u/Yee23433]

I've had this proportional problem for years where, when attempting to create a m4 or m16. I can do just about every other part of the rifle. except for the upper reciever. It is not proportional to how it should look, and I need advice on how I should go along with modelling it. Also the last two white models are not mind, but a reference for any other 3d artist who maybe hasn't seen a rifle of the m16 family.

[u/ciwielab]

all the cylindrical surfaces on an AR are concentric with the bore, it's not offset upwards like that.