If you're using a jig from somewhere like 80% Lowers, or similar, and using your standard router, you have to go slow and be patient. Having done one this way (which is extremely cost effective compared to an actual professional machine) it takes some time, but in the end pays off if you aren't willing to spend the serious coin to get a CNC machine. Also, from a legal standpoint, if you do not personally machine the lower yourself, you have to have the person doing it serialize the lower and do the proper transfer.