Flashback ... June 2006 ... 3rd international sport blowgun competition. New World Record set at 198 out of 210 points! In checking - he used the spearhead darts 0.40 cal. instead of the target darts. (Our bad for not more closely examining - had to alllow it - "Old Wayne" from Lynchburg, VA.). The issue was these dart wires were longer, therefor heavier, therefor had a greater mechanical advantage over target darts. Weight was not taken, but might be 0.1 gm. difference.
Again - a reason to be be very specific about equipment. You want to be comparing "apples to apples".
However, this raises a valid point of discussion ... if you just proortionately size up the target darts, then a 0.625 cal. darts should weight 1.25 gm, not 3 gm. Since the tailcone design by CS is so poor, a light dart like that will never it the target from the standard distance. The tailcone of the 0.625 (new design by CS) is improved, but still not optimal for that caliber. A lot of homemade tailcones look like the film cone darts ("YA") of Japanese design. With a longer tailcone, a longer shaft (for front stability and balance) is needed.
Place a standard 0.40 cal. target dart on your finger to balance it. Note the balance point. Do the same to the 0.50. The relative distance from tip to balance point should be about the same. When you make homemade darts for the 0.625, keep that in mind.