Firearms: Real World to HERO Conversions

The first step is determining weapon damage. Converting the damage of real world weapons into HERO System is four step process. In order- determine Base Damage, Armor Resistance, Penetration Modification of Base Damage, and Stun Modifier

Note: I always use a base weapon cartridge for the calculations. This should be the normal FMJ (full metal jacket) version of the bullet.  Modifications for special ammunition types such as Armor Piercing, Hollow Point, Tracer, etc are handled separately. 

The second step is to convert weapon handling, Five values are needed in this area: STR Min, Per Mod, CQB Mod, RMod, and OCV Mod.

For each step, we'll use the M1911A1 .45 ACP pistol as an example.

Damage Conversion

1. Base Damage

Since the most basic element of damage is how big a hole you punch in something, base damage is determined by the diameter (caliber) of the weapon according to the following chart:

Damage
Caliber	Metric	DC	Damage
0.5	1.27mm	4	1d6+1K
0.1	2.54mm	5	1 1/2d6K
0.15	3.81mm	5.5	2d6-1K
0.2	5.08mm	6	2d6K
0.3	7.62mm	7	2d6+1K
0.4	10.16mm	8	2 1/2d6K
0.45	11.43mm	8.5	3d6-1K
0.5	12.7mm	9	3d6K
0.71	18mm	10	3d6+1K

Example: The 45 ACP has an actual diameter of .452 inches. This results in a base damage of 3d6-1 RKA

The chart can use either inches (Caliber) or metric measures for the lookup. Damage is given in both Damage Classes (DC) and actual RKA values.

A note on the half DC values (5.5 and 8.5): In Hero System 3d6-1 and 2 1/2d6 are equal in active point cost despite the fact that 3d6-1 has a slight edge in final result. The 5.5 and 8.5 values take this into account by displaying the small but significant difference where they are used.

2. Armor Resistance

The most basic measure of penetration is determined by the mass of the penetrating object divided by it's impact area (call Sectional Density in ballistics) multiplied by its velocity. This conversion makes use of the Armor Effect house rules and the Piercing rules from 5th edition Dark Champions supplement to Hero. 

Relative penetration ability can be expressed as:

Mass (in grains) / 7000 / (bullet diameter) ^ 2  *  Velocity)

The result is referenced on the following chart to determine the Hero System Effect:

Example: The .45 ACP bullet has a Mass of 230 grains, diameter of .452 inches and velocity of 850 fps. The Penetration value is 230/7000/.452^2*850 or 136 resulting in an Armor Resistance of x2.

So far, the .45 ACP damage would be expressed as 3d6-1 RKA, Armor x2.

For Penetration values of less than 450, the effect is an multiple of the armor value. For values of 600 and higher, the effect is a reduction of the armor by the listed value as per the Piercing power.

Thus a flak Vest with 5 points of resistant defense would give 10 points of armor against a Penetration 100 weapon, but only 1 point against a penetration 950 weapon.

3. Penetration Modification of Base Damage

Low penetration can result in the bullet failing to inflict as much damage as possible. Consult the following chart and adjust the Damage Class (DC) as indicated:

4. Stun Modifier

These value has the least real world backing as it's related to the old concept of 'knockdown power' which has been debated and argued for generations without a good resolution. 

At its most basic level, there is little doubt that hitting someone with something bigger, heavier and faster produces a better result. To this end, let's use Hatcher's Formula developed in the first half of the 20th century as a measure of 'knockdown power' to determine the Stun Modifier.

Hatcher's Formula (for a Round Nose Full Metal Jacket bullet):

(Velocity * Mass (in grains) * (Caliber / 2) ^ 2  *  3.14159265) / 490) *.9

The result is referenced on the following chart to determine the Hero System Effect:

Example: The .45 ACP bullet has a Mass of 230 grains, diameter of .452 inches and velocity of 850 fps.  850 * 230 * (.452/2)^2 * 3.14159265 / 490 * .9 =  56 or a Stun Modifier of +1.

Thus the .45 ACP damage would be expressed as 3d6-1 RKA, +1 Stun Modifier, Armor x2.

The Stun Modifier only applies if at least one point of body as penetrated the target's armor.  If not, the Stun Modifier is consider to be zero as the bullet bounces off its target.

A modifier of +4 is the maximum allowed Stun Modifier.

Weapon Handling

STR Min

This value in HERO is the STR required to properly handle a weapon. Like most things, a great many factors weigh into this single concept. Shape and design of the weapon, build of the shooter, even air temperature and altitude. Luckily for us much of this would fall between the limited number of values HERO provides. Even so, we're going to have to simplify greatly and ignore important considerations just to be able to produce a single value that applies to all characters without exception. 

Towards that end, we'll focus on a simplified model- that of Free Recoil Energy. Even here, we'll take huge steps of simplification- assuming for example that gas pressure is constant for all weapons (not true in the least of course, we're using the standard value for 30-06 style rifles). Also simplified is the effect of barrel length for which we'll assume that gas pressure is in direct nearly proportion to barrel length (to which we'll add a vastly simplified 10" in order to prevent division by zero error as well as fudging the initial pressure build and the effect of progressive powders). The final number will be a simple measure of Recoil- lacking in many ways, but at least capable of providing us with a result that doesn't require massive research for each individual weapon. Those interested in a full treatment of Recoil are advised to consult Hatcher's Notebook (ISBN 0-8117-0795-4) one of the definitive works on firearms.

The simplified Formula (with various constants left intact) are as follows:

Recoil Impulse (RI) = (bm+(1.75*30/(bl+10))*cm)*15.3*v/(7000*gm*2.2)

Recoil Energy = RI^2*1/2*gm*2.2/32.16

 

bm      Bullet mass in grams

bl        Barrel Length

cm      Powder mass in grams if smokeless (divide by four as rule of thumb for black poweder)

v         Muzzle Velocity

gm      Mass of unloaded Weapon in kg (determining recoil for the last shot, or worst case)

1.75    Constant for 30-06 rifle gas pressures

Once one has determined the Recoil Energy, consult the following chart to determine the base STR Min. 

Example: The M1911A1 pistol has a unloaded mass of 1.106, a bullet has a Mass of 15 grams,  a powder mass of .33 grams, a barrel length of 5", and velocity of 850 fps. It is also a 1 handed weapon..

RI = (15+(1.75*30/(5+10))*.33)*15.3*850/(7000*1.106*2.2) = 12

Recoil Energy = 12^2*1/2*1.106*2.2/32.16 = 5.5

Thus the STR Min for the M1911A1 would be 8 + 3 = 11

 

STR Min Weight Modifier

Very heavy weapons can be unwieldy in their own right requiring a fair amount of STR just to aim and move. It's very common for such weapons to include bipods, tripods, and even heavier carriage mounts in a few cases to offset their mass.

Use the following chart to modify the STR Min determined above:

PER Mod and CQB Mod

These values represent how concealable the weapon is (PER Mod) and how easily it's used in close quarter combat (page 193 of 5th Edition Dark Champions). We'll simplify by basing these values on the overall weapon length as per the following chart.

Example: The M1911A1 pistol has overall length of 8.62 inches give us a PER Mode of +2 and CQB mod of +0.

RMod

The Range Modifiers (bonus to offset Range Modifiers) are based off barrel length as per the following chart. 

While in reality a large number of other factors would apply, to keep things manageable we'll consider only two. Bolt Action rifles by nature tend to be more accurate gaining an additional +1 modifier to the table results. Tube feed weapons (typically lever action guns from the 'western' period) on the other other hand tend to have more problems over range than other types of rifles and as such take a -1 to the table results.

Example: The M1911A1 is a semi-automatic pistol (neither bolt action or tube) with a barrel Length of 5". It has a RMod of +0

OCV Mod

This represents the most important element of handling, just how difficult or easy is it to hit with this weapon period. This is the most subjective of the values and as such should be determined by the GM for the individual weapon.

For myself, I tend to give a +1 OCV for single action weapons of fine quality. Exceptionally well made double action weapons (the Colt Python is a good example) may also be deserving of the bonus.

Two handed weapons are generally given a +1 OCV unless the individual design points out an exception. The AK-47 for example is a very reliable weapon, due in large part to very loose fitting parts- a factor that makes accuracy with the weapon something of fluke. As a result the AK-47 is left without the +1 OCV modifier.

Example: The M1911A1 is single-action automatic known for exceptional handing and accuracy. It's given +1 OCV

Complete Example: M1911A1 Pistol: 3d6-1 RKA, +1 Stun Modifier, Armor x2, STR Min 11 (one-handed), +1 OCV, +0 Rmod, PER +2, CQB +0

Further Example Weapons:

    Modern

    World War II

    Western Era

Final Result

How Realistic is the result?

The short answer? 

Not very.

Just to pick on one of the values above, Hatcher's Formula was meant only to give a ballpark figure and certainly wasn't designed to be used for rifle velocity weapons. In addition, it was a combined value covering penetration and damage and not just a 'Stun Modifier'.

As the tumbling example in the theory article shows, the effect of a weapon is very dependent upon factors not even considered in these house rules- range, bullet construction, target makeup, etc. All left out in the name of simplification. One can min-max the above in silly ways rather easily (thus one should confine conversions to real world weapons) and it's still after all this work a grainy system.

Lastly, the break points in the above charts were selected more with an eye towards creating in game differences between major cartridges than consideration of their actual relationship to real world damage. The relative differences are present, but that's about all one can say about the actual game values.

Even so, the basic concepts I needed are achieved. The results of firearms are now closer to real world expectations and without a complete overhaul of the system. Weapons are better defined and useable in genres ranging from gritty street campaigns to four color comics. Only the most fanatic gun freaks have room to complain.