I seem to recall a discussion surrounding just how tall the checks had to be. Something along the lines of whether the front of the check obscured the gas port sample hole before the rear began to expose it as the bullet passes by. That of course depends upon where the port is located. Rifle length systems have the biggest hole (=taller check). Mid-length a little smaller. Carbine smaller still.
Like Al, I don't remember the particulars but if you use Google's forum search tool you can probably find the thread easily. Its more effective than our built-in search feature here. I elaborate on that in
This Thread in my Information Depository sub-forum here. "gas check" being a good search term.
Hoot
BTW, Welcome Aboard as well!