Material, construction, barrel diameter, wall thickness, barrel profile, gas port location - all play into gas port diameter. I'd have to guess that "experimentation" must play a role in it, too, since there are so many "ranges" in diameters on like barrels. I just worked on a DPMS 7.5" 5.56 barrel tonight. Wouldn't cycle more than 2 or 3 rounds at a time. Took everything apart, and the gas port diameter was 0.052". Drilled it up to a 0.064" diameter, so we'll see how it runs tomorrow. <dontknow> Went through alot of different tests prior to taking it apart, so don't just jump to drilling gas ports - I tried everything else that I could think of first.