Depends on the printer with a high end printer you can print a lower and go with no work needed other than to drill the holes for the FCG and magazine and bolt catches.

With a production or industrial prototyping printer you don’t need to do anything than drop your parts in.

Heck it wouldn’t surprise me if in a few years with multi material printing you’ll be able to print a fully functional lower even with the FCG I’ve already seen metal parinting demos that can print both rigid and spring metal materials in the same print.