Here's a second attempt, more in line with Champsguy's method. There's a good bit of handwaving involved to make it "feel right."
Rough Damage Correspondence Table
Megadamage Hero DC's Points Highest KA Rifts Example Hero Example
1d4, 1d6 3-6 15-30 2d6 Light pistol Derringer, Luger
2d4, 2d6 7-8 35-40 2 1/2 d6 Medium pistol .357 magnum
3d6, 4d6 9-11 45-55 3 1/2 d6 Heavy pistol, medium rifle .44 Magnum, AK47
5d6, 6d6 12-15 60-75 5d6 Plasma cannon
1d4x10 - 1d6x10 16-18 80-90 6d6 Railgun burst, small fusion block Heavy machine gun
2d4x10, 2d6x10 19-21 95-105 7d6 Heavy railgun burst Light tank gun
3d6x10, 4d6x10 22-27 110-135 9d6 Glitter Boy Boom Gun Heavy tank gun
Looking at this, I'm pretty happy with most of it, but the damage on the lower and upper ends still seems a bit off. I'm pondering dropping the +3 DC for megadamage, since 2d6 RKA seems excessive for a light pistol. On the other hand, I do like the idea of lethal combat.
A note on the plasma cannon in question. In Rifts, it's pretty underpowered, IMO. There's a rifle that does more damage (1d4x10 as opposed to 6d6).
And here's a quick look at defenses. The average CS soldier wears 80 MDC body armor, which would translate to 9 rDEF by this table. He uses a 3d6 or 3 1/2d6 RKA weapon.
Body Armor Conversion Table
MDC Resistant Defense
30 4
40 5
50 6
60 7
70 8
80 9
90-100 10
110-120 11
130-160 12
(Edit: added armor table)