Hello everyone,
I am going to guess that the reason that there are no range 21 or 27 weapons is that the range 24 & 30 are optional, and the ranges of 21 and 27 were not needed.
All of my secondary weapons have a range of 20.  I achieve this by giving them range=30 & carronade.  This gives them only short & medium range.  All of my capital ships have range=30 hexes on their heavy weapons.  This makes firing all the weapons of these ships easy to use because they  have the same short & medium range to hit numbers.  And the r=30 heavy weapons on the DNs, BBs, & BCs also have the corresponding long range.
The ranges are 10/20/- for secondary weapons, and 10/20/30 for the heavy weapons.  If I played with players who insisted on using a max range of 18 hexes, The above numbers would change to 6/12/- for secondaries and 6/12/18 for heavy weapons.
I turned 50(!) in Sept of last year and need to simplify whenever practical...<LOL>
PS:  The gamers I play with in south Florida are not going to use the "+3 Piercing" weapon ability any more.  :idea: 
This option makes level shields no more effective than level 4 shields.  Even level 5 shields only block 1/3 of hits from weapons with +3 Piercing. 
I must admit that I have had most of my capital ships outfitted with +3 Piercing on their heavy weapons. :shock:

Hmmm...
While you're clearing up the multiplier for range 21, what is it for range 60?
For some reason I'm not finding it.

RANGE 60 = ORAT x 1000000

Is the changing ORAT the reason weapon range is in multiples of 3? I'd assumed that it was to avoid min-maxing/munchkinism. (It would be easy enough to divide any weapon range--say, range 10--into thirds, rounding off.)

Nope. Weapon ranges are in multiples of 3 just because of the easy division.

Theoretically, you could choose any range value and the game shouldn't break.