I think I read in a book that the gas port should be cut in a groove and not on the land. Anybody have any technical info on this? And why?

I never heard this before.  My guess is that it might not have as much pressure bearing against the bullet surface, and so it might scrape off less guilding metal..??   Maybe it obturates better when the bullet pases by the orifice. Maybe just a stupid guess...my $000.02

How would you be sure its in a groove before drilling?

I think once they get the barrel extension in place the index pin would be top dead center and you measure back the correct distance drill for the gas port.

I could be wrong.

I'm not sure how they do it. I'm sure that technically it could be done. I just wonder what effect it has and why someone would say it makes a difference. I figured that in the land the bullet jacket might scrape more, but that was my $.02 also.  Believe I saw it in the Zediker book Competative shooting AR15.  Looking for someone that could explain that one so a non-tech like myself could understand it.

I'm sure that Kreiger, Shilen or Douglas could get the gas hole in the right spot. My question is why? Or if you are cutting barrels and you cut @ a Land n Groove line (ie part on land part on groove) wonder what that would do?

I have 1 barrel on a land and 1 that looks like it's on the edge.