Saturday, June 27, 2015

Guns Ballistics for Video Gaming

(subject to editing - this was done with out a lot of planning, just a lot of musing while playing)

I've been on a binge of Fallout recently.    Specifically Fallout 3, and Fallout New Vegas, which coincides nicely with the Announced release of Fallout 4 for November 10.

One of the things that's always bugged me about weapons in games is the odd damage values they assign.   Now, I've been reloading for a long time (since about 1973) but I don't consider my self an expert, just a reasonably well informed amateur.  Given that games do a lot of simplification anyway that's probably okay.  What I'd like to do here is lay out some basic concepts for game designers on firearms in gaming.

I notice damage and ammo inconsistencies more in Fallout than anywhere else, mostly because you can make your own ammo in Fallout (it's also my favorite series of games). You have .44 Magnum handguns that do more damage than 7.62x51 rifles.  There are 5.56 sniper rifles that do more damage than 7.62x51 FN-FAL (FWE mod).  The Browning BAR does less damage per shot than the M1 Garand, yet both are .30-06, with similar barrel lengths.  The .308 is more powerful than the 7.62 - which is usually wrong (at least in the FWE mod).   Some of the .308 weapons out damage the .30-06 weapons.   There's a .223 pistol (that uses 5.56 ammo)  but the 7.62 weapons can't use .308 and vice versa.  Who the hell makes a .308 military sniper rifle - it should have been a 7.62.

Yes - most people think the .308 and the 7.62x51 are the same thing, so there's much argument about the interchangeability of .308 and 7.62.   The reality of it seems to come down to three things.

Head spacing: For what ever reason, military ammo hold spec quite well, while the chambers is a lot of military small arms are wildly out of spec - by as much as 10-15 thousands or more.  And yes that's a lot. If you're chamber is excessively long you can run into case expansion, primer backing and case separation issues on ejection.   The military solution for this is to use heavier cases (thicker walls - less internal volume).    If you know your chamber is to spec, then either .308 or 7.62 is fine.  If you're not sure - I'd stick with 7.62 or load your own.  If you're wondering what the hell head spacing is, and why it's critical to the correct operation of a firearm, the wiki description of Firearms Head Space is pretty good.

Semi auto and  full auto have tighter specs requirements than bolt action rifles.  OAL and head spacing as well as overall pressure are specified to provide for reliable cycling of the bolt.  My personal experience is that Federal Match .308 works fine, some of the .308 hunting rounds don't.  So my general rule is Match ammo, or Military 7.62 for semi (and full auto).  Pretty much what ever you want in a bolt gun although improper  head spacing can still cause some issues with accuracy and case extraction if your chamber is very far out of spec.  My personal experience (and your milage may vary) is that for my bolt gun setting the OAL depends on the bullet I'm loading but I get the best accuracy when the bullet ogive (the curve of the conical front portion of the bullet) just touches the lands (that's the raised part of a rifled barrel - lands and grooves).  With semi auto I don't mess around with that - I go for the mil spec OAL and call it good.

Primers: Much of the Nato 7.62 is Berden Primed and takes a lot more work to make it reloadable.  Lake City brass is Boxer primed (standard U.S. commercial easy to reload).   For the sake of games like Fallout, I think it's fair to assume it's all Boxer Primed.

Pressure: For reliable cycling you probably want to avoid rounds that for what ever reason are not very close to mil spec.  It's unlikely that you'll achieve a case pressure in a .308 that the 7.62 brass can not handle as the Lake City brass is actually more robust (smaller case volume, thicker walls) than a lot of the commercial brass.   Again if it's a bolt gun, as long as you're not experiencing overpressure indicators - pitted or backed out primers, cracking of the brass, etc  then you're unlikely to be firing a gun who's chamber can't handled the pressure.   Interestingly where this can be a real problem is the older WWII .30-06 weapons.  You want to really make sure the ammo is not overpowered for those.  Or if you want a really extreme example, compare a old trapdoor .45-70 government load to what you can use in a modern day Marlin or similar.  The stuff I shoot in my 1895 Cowboy would eventually crack the chamber of an early period piece.

For Semi/Full auto: OAL (Overall Length) is more important that for most bolt fed guns because of the mechanics of stripping a round of the magazine and pushing it up the ramp.  A round with an excessive OAL my not even fit in the magazine, and yes I've seen that happen.   Longer OAL is typically a result of loading a heavy bullet or one with a particular profile Spitzer point boat-tails (SPBT) in the upper ranges of weights for a particular caliber will typically be longer than a military ball ammo.   Also not a big concern for gaming.

So what issues should game designers consider when setting the damage rates on their firearms?


  • Bullet Mass: Heavier bullet typically have better penetration despite the fact that they are usually slower than their lighter counterparts.    In a round like the 5.56  you can find loads with 35 grain bullets all the way up to 70 grain.  A 35 grain load is moving about 4,000 fps out of a 20" barrel. While the 70 is going to be moving at somewhere between 2,700 and 2,900 fps (more or less).  There's not much difference between a 9mm 124 grain and a 9mm 145 grain in terms of terminal ballistics.  But when you get to rifle velocities it can become noticeable. 
  • Case Size/Caliber  - This will vary with caliber designation.  Generally speaking bigger is more powerful - but not always.  For example a 10mm is more powerful than a .45 ACP.  The 10mm makes up for it's slightly reduced diameter with extra velocity, bullet masses are similar although on average the .45 ACP bullets are 40-50 grains heavier, you can run a 180 grain HP in both and the 10mm is going to hit noticeably harder.  A typical example would be the .357 Magnum - which in revolver format can shoot .38 special and .357 - both of which have different case lengths and significant different specification for maximum case pressure.  A slightly different example would be the 9mm Largo (9x23) and the 9x23 Win.  The 9x23 Win has a very strong case is rated for MUCH higher pressures - giving it essentially identical ballistics to the .357 magnum.  The largo has a much lower power. 
  • Bullet Composition:  Generally (but not always)  AP rounds will be lighter than the same length ball type bullet.  This is due to the steel penetrator that makes the core of the bullet being a bit lighter than lead - it's not enough to worry about in general.  For example, 5.56 Ball is 55 grains and the Green Tip M865 (standard Nato military) is 62 grains.  Where things get interesting is in the bigger rounds.   For my .45-70 I have loads that run 300 grains at 2,300 fps to the 430 grain moving at 1840 fps.   That 430 grain load will handle pretty much anything on 2 or 4 feet that walks on this planet - although I'd rather use something bigger for rhino, hippo, or elephant.  
  • Muzzle Velocity: All cartridges have a target muzzle velocity - it's the standard round from a standard barrel - for example 230 grain ball ammo from a 5" barrel in a 1911.  Subsonic rounds need to not exceed about 1000 fps (depending on altitude and temperature) 1116 fps at 59F or 15C at sea level.  So 1000fps is a reasonable target.   What this means is that most pistol rounds  for subsonic use will be under powered.  9mm is close, .45 ACP is fine,  10mm will be significantly under powered.   So will all the rifle rounds.   The .300 Blackout (nee 300 whisper) was designed to run both supersonic and subsonic.   The standard round is 7.62x39 and around 125 grain bullet.  The subsonic is about 220 grain.  If I had to choose between a 9mm sub gun, and a .300 blackout SBR, it would be a tough choice - a good expanding 220 grain bullet designed for subsonic use should expand just as well as a 9mm or a .45 but it's much smaller diameter still makes it somewhat problematic, and with no velocity advantage, the lighter round is probably the better choice.   For me the ideal sub gun is the .45 ACP - a full power or even +P round is subsonic so you lose nothing.   For sniper use, there's little point in worrying about subsonic ammo at all - you just don't have the power and accuracy that you need for the job, so stick to regular (or match) ammo.  The suppressors job on a sniper rifle is to make it harder to locate your position, not to keep anyone near by from knowing rounds have been fired. - Keep in mind, a really good suppressor is only going to drop the sound of a rifle to somewhere around 120-130db and that's not quiet - subsonic rounds will be noticeably quieter but nothing is silent - an MP5-SD will sound a bit like someone saying CLACK CLACK CLACK at slightly above conversational levels.  A subsonic 22 will be the quietest you'll come up with. 
In general - you want to look up muzzle energy of the various rounds you'd like to use in the game, and assign a base damage to each caliber based on that.   You can further assign damage modifiers for hollow point and armor piercing - HP doing more damage to unarmored targets but significantly less to armored targets.   Using the Damage Threshold from Fallout 3: New Vegas  a standard round would have no DT modifier, a HP would have a small but noticeable positive modifier and AP round would have a noticeable negative modifier - but would also have a somewhat lower base damage than the 'standard' round as it will not expand or deform as much.    This will prevent the bizarre cases where a handgun does significantly more damage than a rifle.  Sure there are cases, a 22LR rifle will do less damage than .38 special  - maybe even a .380 auto (9mm short). But scaling on muzzle velocity will make things consistent. 

For guns, we need to consider four things.
Barrel Length: which has a couple of  interesting effects - short barrels will reduce velocity, standard barrels will have no effect and longer barrels will also reduce velocity.   Yeah that seems weird but think about what's going on inside - when the powder burns, it creates a rapid change in pressure as the gas expands.  The powder will continue burning typically while the bullet travels down the barrel.  Ideally your powder will completely burn just as the bullet exits the barrel.  With short barrels (compact hand guns, SMG's etc) the powder will still be burning after the bullet has exited the barrel - that energy does nothing to accelerate the projectile.   The same round, in a long barrel for the sake of argument here let's use a .44 Magnum revolver with barrels of 3", 5" and 10"   Our 3" will do somewhat less damage because the same bullet is moving slower, the 5" the standard to which the ammo was loaded will have no effect.  The 10" barrel how ever provides 5" of barrel friction with little additional acceleration so. It's likely to fall somewhere between the 5" and the 3" barrels for velocity - you will gain some extra accuracy due to a more stable exit from the barrel, and the longer sight radius (assuming no scope).   If we look at the 1911 style .45ACP  a 3" compact will have a muzzle velocity some 100-200 fps slower and when we're talking about a 900fps standard - that's 10-20% slower. That's a lot.   For a 5.56 - a 8.3" barrel (SBR short barreled rifle) will loose 130 fps over the standard 20" barrel that the round was designed for.    If you were silly enough to make a 30" barrel for a 5.56 it would likely lose even more (although I have no way to test that).

+P rounds will give that 3" barrel about the same velocity as a standard round in a 5" barrel.

A .22LR rifle with a 14-16 barrel will do better than a 6" pistol but for game purposes probably not a lot - maybe 10% or so.

There is no significant difference between .223 and 5.56 as they are the exact same bullets moving at very similar velocities.  The same goes for .308 and 7.62 - although the .308 is more likely to have a heavier bullet - say the 180gr SPBT.  The 7.62 is typically the 150gr steel core lead bullet (AP)  or the 168 grain Match ammo.

SMG's and pistols should do about the same damage with the same rounds.   Suppressors will not affect the velocity much at all, assuming a non-specific sub-sonic round.  And they typically provide a very slight increase in accuracy (spread - like maybe .2 MOA)   That additional accuracy when it happens is generally attributed to the bullet stabilizing earlier in it's flight path due to less interference  of gasses escaping the muzzle.

Typically, shorter barrels will have Higher MOA - minute of angle - essentially 1MOA is 1" at 100 yards (close enough).   So more spread.   A typical bolt action will have less than a semi.  and Full auto will increase the spread significantly.  Burst fire 1 MOA might easily become 6-12 MOA. Long bursts without a lot of skill will see MOAs of  30-60 inches at 100 yards.   A really skilled operator can probably keep 15 rounds on a 10 plate at say 10 yards  - at 50 there's going to be a lot of missing. There's not much accuracy to be gained by going from a 16" barrel to a 20" barrel.  For .308 - some experts find that you might was well use 16 or go all the way to 27.

Type of action: Bolt action is going to be more accurate than an auto-loader. Although, I can show you specific examples of a semi-auto being significantly more accurate than a bolt gun - it's pretty much always a quality issue.  For handguns, it's a bit of a wash - you can make both types of handgun more accurate than the shooter so... In terms of muzzle velocity, an auto loader will have a higher muzzle velocity than a revolver because there's no leakage of gas pressure, revolvers have a gap between the chamber and the barrel that allows some significant amount of gas to escape - in the case of the really big revolvers the .50 AE or the .454 Casull - enough gas can escape to literally amputate a misplaced thumb. Don't get any part of your anatomy forward of that revolving cylinder or you might lose it.  In anywise - it won't significantly affect the damage as there aren't that many rounds that are used in both revolver and autoloader.

Rate of Fire: This will mostly affect accuracy - the DPS is simple math assuming the rounds hit the target the question is - will they.   Higher recoil (more muzzle energy) can be controlled by skill to some extent, and weight/balance.   The addition of compensators will also help this.   The advantage to very high rate of fire (ROF) is really in the short burst.  the muzzle doesn't really have the time to climb much between rounds.  A 9mm sub gun with a full 30 round magazine can be held on a man size target at 20-50 feet but it really takes skill and strength. A heavier gun will have less issue with this, up to a point.  A heavier gun is also more difficult to hold steady (think Light Machine Gun).

Quality:  In terms of accuracy this - given the same ammo - is the biggest factor in accuracy.  A very high end bolt action sniper rifle will run 3/8 MOA - that's almost overlapping holes at 100 yards for a .308 or .75 inches at 200 yards - which seems to be longer than any shot you can achieve in Fallout - due to (I assume) graphics considerations.

What does that mean for the "unique" guns?  from a "realistic" point of view - I'd rather seem them have much improved accuracy, rate of fire, and spread than noticeably more damage.  Perhaps though the perfect balance of weight, trigger, grip, recoil mechanism and other factors  that 10mm Ultra SMG will be able to do a much better job of keeping those rounds on target - but having 30% more damage - well I gotta wonder where the heck is that extra energy coming from?