It sounds like you were told they would fit based on the predication that they are meant to function with a rifle length gas tube under them, when used on an AR15. What most people may not realize is that while plenty of the AR10 barrels on the market use a standard rifle length gas tube, the gas port distance is different than the standard distance on an AR15 with a rifle length gas system.
The difference in distance is to account for the fact the barrel extension on an AR10 is longer than that of an AR15, subsequently, the gas port distance on the AR10 is shorter... to account for the increased length of the AR10 barrel extension.
So if I drill a port 6mm further the gas tube will be too short then. Man that sucks gotta figure out then.