I really want to run a Cyberpunk 2020 game, but I hate cyberpunk's combat system. Is there an easy way to implement D20 modern style combat into Cyberpunk 2020?
I really hate the damage soak stuff, and I really like the straightforward nature of D20 modern and 3.5 combat systems. How hard would it be to take the weapons, items, mods and stuff from Cyberpunk 2020 and convert it into a D20 modern system? Specifically damage system, initiative, armor and maybe feats.
Best Answer
In short? Not directly, and not easily. In long... any numeric system has a scale, and while you can develop rules to rescale things, you need to be willing to break a lot of conventions, and you're going to have to houserule things.
I'm assuming you want to use the CP2020 system by and large - lifepath, chargen, criticals, fumbles, luck, the actual die roll itself, et multiple cetera. I'm limiting my answer to only changing the combat mechanics, rescaling numbers from one system's mechanics to another, and some notes about the differences in lethality - you're going to have to make some calls.
Damage System
Your main point of contention, Cyberpunk's Stopping Power mechanic, points to the a core difference between the two RPGs. Cyberpunk's Friday Night Firefight system is based on real world energy values, and armor slows impacts but doesn't stop you from getting hit... at least, until it's been ablated enough. d20 Modern is based on cinematic lethality: shots that hit armor either ping off to one side or shoot right through and bury themselves in meat for massive damage.
However, we can cheat. Both systems have two assumptions about how damage work which are near-identical, and because of that creating 'benchmarks' for us to use, we can do some conversion.
First, ammo determines damage, with very few exceptions; a .22 round will do the same damage from one d20 Modern weapon to the next, and it'll do the same damage from one CP2020 weapon to the next, with few outliers that could be represented as a special case. For example:
d20 Modern damage starts to become more 'compact' at the far end of the bellcurve:
(Notice that d20 Modern almost always uses 2dX for ranged weapons; this is strongly imposed by weapon feats.)
Second, both systems average a similar amount of damage until collapse - not death, but collapse - but for different reasons.
In d20 Modern, if you take damage equal to your massive damage threshold, you have to make a DC 15 save to stay standing; fail and you pass out. This massive damage amount is your Constitution score; so for an average person, 10 or so. This will take you out about 70% of the time.
In Cyberpunk 2020, you take stun saves for every blow you take - rolling your BOD or lower on 1d10, with a -1 penalty for every new tier of damage. A 12-point blow will drop you about 80% of the time. (12 points damage, -2 BTM; 50% chance of passing the first save, 40% chance of passing the first save = ~20% chance of succeeding both.)
Soooo... despite having completely different aims, the damage systems are convertable. It wouldn't be a simple mathematical trick, but it could be done.
What you'll have to do
Create the damage table. CP2020 weapon on the left, d20 Modern block on the right. (It'll be worth it to have firearm and non-firearm damage effects on different tables, because non-firearm damage effects are scaled differently due to working with feats differently.)
Come up with a rule for a massive damage threshold. BOD+7 could work; the low end wouldn't be as low as d20's low end, but the average and high end would be about the right scale.
Come up with a rule for hit points, so you can model damage that accumulates over time. I can't give you a hard and fast rule, but taking your massive damage threshold and multiplying by 2 or 3 is what I'd go for; in Cyberpunk 2020, even with armor, you can't take too many blows doing 10+ damage before you keel over, armor or no. (This also models the fact that armor degrades over time in Cyberpunk, something that d20 Modern doesn't model... and by your distantes for the Stopping Power mechanic, I assume it's also something you don't want keep track of.)
Come up with a rule for converting Structural Damage Points to hit points for non-characters. I'd suggest HP=SP for vehicles for simplicity's sake... or just dropping hit points entirely, since d20 Modern doesn't worry about equipment damage much.
Skill Bonuses
d20's various maneuvers, for the most part, just 'do what they do'. The biggest thing you need to convert are skill check bonuses.
For d20 Modern characters, your average untrained skill check result is 10: +0 attributes, +0 ranks, +10 die roll. Your maximum possible result for a top-end character, barring special bonuses, is 59: +6 attribute (18 base, +5 levelups), +23 ranks, +20 die roll. That's a 49 step difference.
For CP2020 character, your average untrained skill check result is also 10: +5 attribute, +0 skill ranks, +5 roll. However, your maximum possible result for a top-end character, barring special bonuses, is 30: +10 attribute, +10 ranks, +10 roll. That's a 20 step difference.
What you'll have to do
For converting d20 maneuvers to modifiers that fit the Cyberpunk resolution scale, divide modifiers by 2.5: every +5 in d20 Modern scale is +2 in Cyberpunk scale.
Convert d20 Modern-scale DCs to Cyberpunk 2020-scale Target Numbers. 10 in Cyberpunk and d20 Modern are about the same; after that, every +5 to the d20 Modern DC is roughly +2 to the Cyberpunk 2020 Target Number.
Initiative and Actions
This requires the least amount of work. Both systems are, essentially, move and attack, with movement being based on a movement score and attack being a skill check. CP2020 gives you move for free, d20 Modern has actions that interfere with movement.
What you'll have to do
Are you keeping the Cyberpunk 2020 dodge check, or are you changing to a static Defense-based system like d20 Modern? If so, define Defense. I'd suggest 7+REF: a direct conversion would be 5+REF, but without a Dodge skill or the ability to critically succeed a dodge check, a Defense of only 5+REF would make the already-lethal Cyberpunk combat far more lethal. You don't want to make Defense much higher though, because of Armor...
Armor
This is where the two systems' conceits are farthest off. Cyberpunk 2020's encumbrance values (EV) is roughly half the impact of d20 Modern's armor penalties and max dex penalties (but then, see also item modifiers above). The impact of d20 Modern's encumbrance is actually reduced by having a low dexterity score.
Cyberpunk 2020 stops rounds dead; d20 Modern either works or it doesn't. And unlike weapons, there's no direct comparison of values across the two games, because while both games share a weapon list, the armor in both games is far more abstract - and without equal conversions. The maximum for Cyberpunk 2020 - MetalGear-brand EOD disposal gear, SP 25 - doesn't match its d20 Modern equivalent EOD gear (+9 AC) well at all, decades of development separating the two. Finally, Cyberpunk's encumbrance is a defense penalty. d20 Modern's encumbrance is a movement penalty (as well as the armor use and AC bonuses).
What you'll have to do
This is where you're furthest in the dark. There is no direct conversion, and a lot of GM calls that you'll have to make.
You could divide SP by 2.5 and round down to get rough AC bonus. It's not perfect - Metalgear, with its sniper-round-stopping SP 25, now becomes just a +8 - but it fits the Interlock system's skill check scales.
As to encumbrance, I'd recommend just leaving it as a movement penalty - the Cyberpunk genre is lethal enough without importing the d20 Modern system's AC limitations.
As to armor type, you can define armor type by the AC bonus they grant. In this way, Cyberpunk's Kevlar Ts and leather would be light, armor jackets and flack pants would be medium, and heavy armor and specialized gear would be heavy. EVs could be an indication of armor type, as well; EV 0 is light, EV 1 is medium, EV 2 or 3 is heavy.
Movement
Change the movement allowance to 5' × MV, assume most non-heroic Cyberpunk citizens are a little slower than d20 Modern's TV action hero defaults thanks to one too many Petrochem-brand microwave burritos, and roll with it.
Range
d20 Modern weapon ranges are equal to one-tenth of their maximum usable range, barring feats; Cyberpunk 2020 weapon ranges are equal to one-half of their maximum useable range, barring special setups. As a casual rule, divide Cyberpunk 2020 ranges by 5 and convert from metric to standard to get a d20 range increment.
And remember, each range increment is -1 once you've scaled the d20 Modern penalty to a Cyberpunk 2020 scale penalty.
Further Reading
The RPG system Fuzion, used in Cyberpunk's third edition, does most of what you want already. It's a direct evolution of the Interlock system used in Cyberpunk 2020.
Guns, Guns, Guns! is a document with more information about damage systems in RPGs than you'll ever want or need - but it's helpful for comparing values across systems that treat damage dissimilarly. Since that's the biggest difference between the two systems, I'll recommend it here.