Thanks for the info. I have put on order an 18" barreled upper with a DPMS pattern lower. This may not be the most precision a person can get, but I'm going to see what it will do.
Here is another article about barrel length that guided me toward this decision:
The Truth About Barrel Length, Muzzle Velocity and Accuracy - The Truth About Guns It was an eye opener for me. I would always have fallen on the side of longer being better, but now I'm not so sure.
Once I get the whole thing built, I'll do some shooting and see what we see.