Converting GURPS weapons to d20

[HeavenShallBurn]

I'm sitting here with GURPS Ultra-tech open. Mostly I'm wondering how to appropriately convert this very nice system of future-tech weapons for use in d20. Note don't suggest d20 Future it's just not up to par IMO.

See I'm considering a weird interstellar/planehopping campaign set in the future that freely mixes science (both hard and soft sci-fi in various areas) and high-magic ala standard D&D. But the GURPS weapons are much better than d20 Future both in design and implementation. Only they go about things a different way. Most of the factors are easy enough to convert, except damage.

Does anyone have any ballparks of how to relate damage between GURPS 4e and 3.5ed&d?
Example 1

Particle Beam Pistol 3d(5) Now since I use an armor as DR variant I figure just directly convert the (5) armor divisor to 5AP even if that weakens it a bit because it's faster. But how should the 3d convert? Given the differences between systems I'm thinking it should go up but I'm having trouble figuring out how much.

 

[HeavenShallBurn]

After consideration I've modified WarlordRalt's firearms system from the NovaWars data that was over in the d20/OGL section a couple years ago. I'm still working out how to apply damage for warheads of projectiles. When I'm fully done I'll post the system in this thread.

Example: gyrojet medium pistol sized for a medium character with no warhead on its shells
5d6\24y\3AP\20x3
Damage is very heavy compared to d20 Future. On the other hand the system is derived from D&D not d20 Modern with it's fairly low damage threshold. The fatal flaw of d20 Modern and its derivations like Future was that they tried to keep melee on par with firearms. Creating a situation where a d20 Modern character weilding a sword could do more damage than a firearm. That is corrected, weapon ranges are also extended to something more reasonable as well, and Spot(Perception) rules are being altered to support longer engagement ranges.

 

[HeavenShallBurn]

These are the basic firearm categories upon which various modifications are added. This is designed using the 3.5e weapons categories and base size medium. The ranges on pistols are a bit generous but those for rifles and heavier weapons are far closer to their effective range IRL than the d20 Modern ranges. Most infantry combat occurs within about 150 meters, in urban combat much closer, this should be handled through spot rules not adjusting the range of the weapon itself. Range increments are handled in yards because for game purposes they're interchangeable with meters. Pistols and Rifles are by default considered SA and can be made A by adding on that property.

Firearm Types
Range|Action|Magazine|AP|Damage|Weight
Light Caliber Pistol (.17, .22L, .38, etc)
10y|SA|12|0|3d6-20x2|.5lb-1.5lb
Mid Caliber Pistol (.357M, .45, 10mmHP, etc)
10y|SA|9|1|3d6-20x2|1lb-2.5lb
Heavy Caliber Pistol (457C, 50AE, 60E, 15mmC, etc)
12y|SA|5|2|3d8-20x2|3lb-5lb

Light Caliber Rifle (.17Hornet, .22LR, etc)
60y|SA|30|1|3d6-20x2|5lb-5lb
Mid Caliber Rifle (5.56, 7.62K, 6mmI, etc)
90y|SA|30|3|3d8-20x2|5lb-9lb
Heavy Caliber Rifle (7.62N, 30-06, 7mm, 8mm, etc)
100y|SA|30|5|4d8-20x2|9lb-12lb

GPMG (chambered as heavy rifle)
175y|A|-|5|4d8-20x2|15lb-25lb
HMG (50BMG, 14.5mm, 15mmSwiss, etc)
300y|A|-|10|8d8-20x2|45lb-80lb

Light Autocannon (20mm-35mm)
450y|A|-|15|12d8-20x2|115lb-300lb
Medium Autocannon (40mm-76mm)
450y|A|-|25|16d8-20x2|400lb-1500lb
Heavy Cannon (90mm-155mm)
700y|S|-|50|32d8-20x2|~2000lb-~7000lb