Well OGL Cybernet is based off d20 Modern and the coarse grainularity of fitting all calibres into 5 types (2d4, 2d6, 2d8, 2d10, 2d12) comes from that. The idea is that the your choice of weapon comes down to style, not the one that does the most damage (though the 5Sevens in Ultra Modern Firearms skew this a bit
). Here is an article by Charles Ryan on the subject (http://www.wizards.com/default.asp?x=d20modern/bp/20030218a
15mm & 20mm guns would be in the 2d12 range (putting it at the same power as a berrata light fifty). Edit:
Just double checked in OGL-C and light fifty does 4d6.
Penetration could probably be modeled by giving armor a form of DR against certain grades of ammo (either by calibre or by damage dice) with armor piercing rounds doing extra damage (thus negating some of the DR) or being treated as a a higher calibre or damage dice.