I'll tell you how I build rifles: I select a rifle configuration that's an issued weapon with US or other friendly forces and build according to its specs as much as is possible.
I COULD just build it any way I pleased, but it's more fun for me to make replicas of specific
versions that are used by our troops and allies.
I've begun acquiring parts for a Mk 12 Mod 1 rifle and intend to start putting parts together
within the next few weeks, if all goes well.
Getting some experience by building standard models is recommended, before you start
off on your own custom version.
I'd recommend starting with a basic M4gery, or A2, A3, or A4 clone, and then branch out from there. Get familiar with the rifle by making a standard baseline design first,
before getting really specialized.
CJ