While I'm generally in favor of careful experimentation, have you compared the external and internal ballistics of the .30-'06 vs. the 6.5-'06? The garand depends on a precisely metered amount of gas to operate the action. Too little, and the thing short-strokes, too much, and the op rod bends, or worse. Some (but not all) of the variables are: diameter of gas vent, location of gas vent (fixed in the garand), volume of gas in the barrel, pressure of gas in the barrel, duration of bullet presence in barrel after passing gas vent. In short, quite a few things to sort out and juggle. Also, if the exterior ballistics of the two rounds are not identical, the range (elevation) calibrations on the rear sight will be useless. Can it be done? Certainly, but at great expense, and a lot of effort and time. FWIW, I'd iron out as many variables as possible and obtain datum points on a bolt-action rifle first. For starters, you'll need a chronograph, and an accurate way of determining pressures within the barrel. Kuhnhausen's book on the US gas operated rifles is a MUST READ. Good luck!