How do I build the lightest possible AR-15?

[TheHossUSMC]

Magpul makes polymer BUIS if you do want them. And IDK who makes them but ive seen polymer quad rails. I don't know if the quality is any good but daniel defense makes lite quad rails but very spendy. Personally I would at least get a m-4 profile barrel. Unless you plan on never rapid firing.

 

[SSN Vet]

16" pencil barrel on a Cav. Arms poly lower..

 

[Zach S]

So it seems that my choices are a bit too limited with 5.45.

Adco firearms can turn your barrel down. Its $95 if you just send the barrel, more if its part of an assembled upper.