The DefCad AR-15 lower download is in .STEP format, and that doesn’t do much good. I downloaded freecad and converted it to .STL format. It sliced for me but I don’t have my printer up and running at this exact moment so I can’t say if it is a legit conversion or not.
Hopefully my printer will be up to the task soon enough. I need to replace my smooth rods and bearings.