Hey! I resemble that remark...
I've been on both sides of Dunning-Kruger. But in this instance I'll bank on my engineering and job experience and claim to be on the right side.
I loved Lego's as a kid. Not much has changed.
The carbine I built is a mid-length version of a Colt 6720. The main parts are a DD barrel, Colt upper receiver and LPK, and BCM for the fixed front sight and BCG. I don't think it's "stupid dumb luck" that it runs.
That being said, I bought my last upper from BCM. It was easy, and cheaper than the parts.