oh damn... I hope it's a joke, not your real argument ? well in case you are not aware of this... your 12gr bullet weight is more like:However, was there ever a bullet that was manufactured that weighed 12 grams
12.05gr, 12.0002498gr or 12.03723952gr (which is closer to 0.78g

....but for simplicity sake it is rounded to 12gr because it was measured and written down in a country that uses imperial units...
damn you really have lame arguments...
well in case you haven't noticed... your 'real' world uses an archaic measurement system...
Indeed, if you are making air guns, it is nice to compare your results with the "real" world
so yeah if you want to you can even post here using hieroglyphs or Indus script

check some sources outside of the USA and UK. No one uses grains any more.The grain is used to measure the mass of bullets, gunpowder, and smokeless powder; it is the measure used by the balances used in handloading; bullets are measured in increments of one grain, gunpowder in increments of 0.1 grains.[3] Moreover, the grain is used to weigh fencing equipment, including the foil. In archery, the grain is used to weigh arrows and arrow parts
In fact I guess that most of gun manufacturers, engineers, basically anyone who has to calculate stuff, etc. would like to switch to grams and metric units becasue they are more convenient... but they can't do so becasue there are lots of hillbilies (metaphorically speaking) who don't know how to use them... not becasue grains are better