Pimp your own gun for MegaTraveller

[OjnoTheRed]

Following on from my musings in this thread, I started toying with the TNE small arms design system in Fire, Fusion and Steel to see where it would take me.

After staring at the designs created and trying to reconstruct existing MT weapons, I think I've got a conversion system going.

It's in this OpenOffice Calc Spreadsheet. Basically fill in the fields with green labels (occasionally picking from a list highlighted in colour) and you've got a small arms specification.

I'm still refining it. I'm going to come up with an arbitrary system for Signature (based on muzzle energy, I think), and convert TNE difficulties to equivalent MT difficulty labels for each range band. I expect the vast bulk will come under the standard Handgun / Rifle / Gyrostabilised difficulties, but I'm going to do it largely as a quality check against assumptions made.

I'd be interested in any feedback. A further development when I've refined my ideas is to convert it to a web-based javascript or PHP based application.

 

[Ishmael]

MT gun stuff

Here are notes for stuff involving converting to MT from FF&S1 I did a while ago, and relating it to a form of the AHL combat table.
( as well as some other stuff for an action point combat system...unfinished)

I hope its useful.
This should give clues to convert between different versions when starting with real-world units for muzzle energy and bullet mass, etc.
A warning though...I am using bullet momentum for damage in order to model the characteristics of big heavy slow bullets versus small fast moving ones.
This seems to give sane numbers for guns like .54 Hawkins or when comparing a 9mm autopistol vs a .45 acp
It might be useful to know that MT attenuation correlates to balistics coefficient

http://www.sfrpg.org.uk/phpBB3/viewtopic.php?f=34&t=58

your spreadsheets sure are nice looking....can't wait to see it once its done

 

[OjnoTheRed]

Hi Ishmael. Firstly, thanks for the compliment on the way it looks. I didn't think it was a big deal and there's waaay room for improvement. My idea is to get it to the stage where after plugging in the specifications for ammunition and weapon design, the equivalent MT stats are automatically calculated on page 3.

Things to do to finish it:

1) Difficulty profiles for the weapon elaborated;
2) Signature and Danger Space automatically calculated;
3) Implement multiple barrels, automatically detecting and calculating ROF for automatic weapons with multiple barrels, including validating for the type of receiver and receiver action selected. As a knock-on, automatically calculate Autofire Targets. For example, if the user selects Breach-Loaded for Feed System, 2 Barrels, with a Receiver Action of Individually Loading, we know they are basically designing a 2-shot gun with ROF1 (probably a hunting rifle or a double-barrelled shotgun).
4) Difficulty profiles will account for advanced sites etc., but there will be a free-text addition to indicate where advanced slug throwing options are used such as optic sites, etc.

... and then things to do to polish it:
5) Move the look-up tables to a seperate sheet just for layout clarity;
6) As a consequence of 4) ensure that where a text option is required (e.g. "Breach-Loaded" or "Box Magazine" needs to be typed in) that valid values are clearly displayed for the user.
7) Use the look-up tables to validate user entries and display meaningful error messages for invalid entries.

I've had a brief glance at the link to the SFRPG forum, but haven't read it in detail so I haven't gone through the debate around bullet velocity vs. weight thoroughly. I'm also by no means a ballistics expert! My firing of guns has been limited to use a 0.22 breach-loading rifle for target practice many years ago with the Scouts. No, I was not very good at it.

But I'll share this observation from the FF&S design sequence. If you increase the calibre of the bullet, muzzle energy (in Joules) goes up with the same cartridge length. If you increase the cartridge length, muzzle energy also goes up. It seems to be me that in TNE, muzzle energy in Joules is the bullet momentum that you are talking about - it's not just muzzle velocity because the weight of the bullet is taken into account in the energy rating.

Is there something I'm missing here, though?

The one thing I've realised, too, about the design sequence is the minimum 10cm barrel requirement seems to me to make it impossible to design very compact body pistols (I'm thinking James Bond style here like the Beretta or Walther PPK) which are for close-range cold-blooded murder work rather than open combat. But again, I'm wondering if I'm missing something? It's only around 30mm I'd need to shave off the barrel (down to 7cm) to get the same specifications as the Walther PPK, for example.

 

[Ishmael]

I'm afraid that although bullet momentum is related to muzzle energy, it is not the same.
Momentum is = sqrt(2*mass*energy) and is equal to mass*velocity

Because of how mass increases by a power of 1.5 with relation to bullet diameter ( approx.), for big heavy bullets, it begins to become a bigger factor than simple muzzle energy at the point of impact. Damage is caused by the transfer of momentum to the target.
Also, a more massive bullet will have a higher attenuation number in MT as it takes loger for air resistance to blend away all that momentum.
The end result is that for a constant muzzle energy, a large heavy bullet will have lower pen, higher Attenuation and transfer more momentum to a target than a slender lightweight bullet.
It will have more stopping power and cause more damage.
This is as veteran pals of mine describe when comparing a .45acp to a 9mm and what I've seen when comparing a .54 hawkins or a 45-70 to a 5.56 Nato.

Bullet momentum divided by gun mass gives a measure of recoil.
Bullet momentum divided by target mass gives an indication of knockdown. Flexible target suffers less knockdown as the flew absorbs the momentum...soft body vs rigid body dynamics and all that stuff.

otherwise, the best gun will be made with a high-power cartridge firing a light, slender dart.

this effect also shows up when describing archery, or thrown spears where the 'muzzle energy' is very low because that energy comes from the archer's muscles and not from a chemical explosion. As that link describes, I figure a character can produce 10 joules per strength point per second and a bow allows those joules to be stored as more are added up to a limit, thus a bow might only give a 'muzzle energy' of 70 to 140 joules ( crossbows can store more although it takes longer to pump energy into them), yet because the arrow is massive compared to a bullet, although it travels much much slower, it can cause decent damage. Its pen is smaller and can be stopped by armor.

this also works for me with melee weapons where damage is often proportional to weapon momentum at impact... axes and hammers have more momentum at the impact point that a sword of equal mass due to mass distribution.... , 'muzzle energy' in this case comes fr muscles and =10 joules per Str per second

thats what I'm modelling , any ways.

I'm going to double check the barrel length issue...and see what I find.

 

[OjnoTheRed]

Thanks for the patient explanation! I think I see the distinction - bullet muzzle energy is what it's got when it leaves the barrel, bullet momentum is how much it loses that energy as it travels?

In short - higher mass bullets do higher damage but penetrate less well and that penetration attenuates at a faster rate?

So what's happening in TNE is that muzzle energy is being used as a bit of an abstraction to calculate both penetration and damage?

I saw the impassioned article about gunshot wounds in FF&S but that really only addresses how a bullet interacts with a human body and what damage it does. The other side of combat in a Traveller setting is how many centimetres of steel can we expect it to penetrate?

What I've done with the spreadsheet is simply calculate penetration and attenuation by asking - at short range, what level of armour is necessary to halve the damage rating of the weapon? Then double it from the TNE rules because 1cm hard steel = toughness 2 in TNE, but = 1 armour level in MT (which gets it from Striker). Then track at what point in the TNE meter-related ranges does penetration halve again against the same level of armour? That gives you the number of range-bands attenuation.

Now, all of those figures are a bit rounded an approximate and lean heavily on the ballistic assumptions in FF&S. I cross checked them by taking the rules for the 7mm and 9mm rifle and auto-pistols from the MT standard rules, and played with the ammunition, barrel and receiver settings until they had about the same length and weight as the MT rules. Then I looked at what damage and penetration resulted. They were close enough, I thought after several iterations of my assumptions. Maybe MT used similar ballistic assumptions to TNE

 

[aramis]

Technically, momentum is velocity * mass. It is the same, in theory, as muzzle energy while the bullet is at the muzzle.... but it drops with friction as the bullet travels.

Penetration (in the real world) is the distance through a uniform medium that a projectile will produce a hole (or cause a spall). It is a function of hardness of medium, hardness of projectile, diameter of projectile, shape of projectile, momentum of projectile (including mass and velocity).

Damage is a fuction of the energy dump transforming kinetic energy into tissue disruption. Larger diameter rounds tend to shed energy into targets better than narrower diameter rounds. More massive rounds have more energy to dump.

Given two rounds of equal energy & mass, the penetration of the narrower one will be higher, but the energy dump will be higher on the wider one. If, however, both have low enough penetration to stop in target, and high enough to go into significant tissue, the net damage should be based more on where it traveller than on shape of round.

(EG: In other words, a .22short vs a .25acp, vs belly, both should dump about the same energy, and should stop somwhere in the intestines, and the damage then matters more which got lucky and nicked an artery or vein, or perforated the intestine...)

Game abstractions for FF&S are that damage is a function of energy alone (ignoring the rate of shed issues), and penetration is energy over cross-section. MT makes similar assumptions, but presumes both a 3 point KE penetrator base damage limit vs people since most will in fact go through people and not shed all their energy... high pen tends to generate more damage in MT.

 

[Ishmael]

I also looked into the 10cm limit you mentioned.
Theere is a limit of 10cm for the "average barrel length"
The actual barrel length can be as short as 20% or as long as 230% of average barrel length
The absolute shortest barrel in FFS1 is 2 cm...less than an inch, but changes from the 'average barrel length' affects muzzle energy vs propellant energy.

Aramis: Yes, momentum =mass*velocity
I didn't care to show each step in how I got sqrt(2*mass*energy), but that comes out of the m*v stuff directly
I do damage this way because the abstractions used mean that the best pen and thus damage would always come from a small bullet and that doesn't model the big heavy slow moving bullets of black powder rifles too well.

 

[OjnoTheRed]

Ishmael, thank you for looking into the 10cm barrel length question. Yes, on re-reading it that way, it's just saying that the starting point for ballistic calculations on barrel length is from 10cm even if the calculation came out less than that. So, snub-pistol design here we come.

The next iteration of the spreadsheet can be found here.

The only remaining feature missing is some form of auto-calculation for "danger space" in Mega Traveller. The rules in TNE are really complex for multi-bullet rounds and bursts. Each shot rolled separately then with cascading chances of lesser hits after calculating a danger space. Must have been unplayable.

Improvements in this version (and I'd like comments on the conversion decisions I've made):

• a TNE evaluation for the weapon (well, given the data is all there, why not?);
• a weapon signature based on muzzle velocity (< 1000 joules = Lo; >1000 joules = medium; >1000 joules and 80% barrel length = high) - I'd like other views on that;
• MT difficulty profiles. CT/MT ranges are compared with the calculated TNE ranges. If the CT/MT range band is anywhere up to 50% higher than the calculated TNE range band, it gets that range bands difficulty. For example, if Short Range for our rifle is 100 meters, Medium Range in MT gets Difficult; Long Range (at 250 meters) gets Difficult (= Difficult or Formidable in TNE) because it's half-way between 200 and 300; V Long (at 500 meters) gets Formidable. Then Distant (which usually falls outside the Extreme range for these varieties of firearms) gets the Impossible rating If and Only If V Long is Formidable AND the weapon has an extreme range beyond V Long. This replicates the MT handgun and rifle difficulty profiles for the expected standard rifles and handguns. There's a bit of abstraction here for the sake of gameplay.
• Calculation of number of auto-fire targets. This is done by detecting the receiver type and action; if a receiver type with ROF 1 is chosen, autofire is ignored. If a self-loading receiver is chosen, then a Selective gets one ROF/Autofire Targets, and a Deluxed Receiver Action gets two different ones (i.e. firer can choose between single shot or two different bursts).
• Recoil in MT - at the moment it's "Med" if it's less than 8, or Hi if equal to or above 8. This is taken from the MT recoil rule that says on Hi recoil the firer must be braced and cannot move that turn. This compares to the TNE rule where recoil is compared to strength and strength-minus recoil is imposed as a DM - which can be alleviated by being braced - i.e. TNE basically gives an unlikely shot a chance. I figure if average strength is 7, then 8 or more becomes high recoil.
• The "/R" rating on recoil: I figure if a weapon has a self-loading receiver, the firer is capable of pulling the trigger until the weapon is empty.

 

[OjnoTheRed]

I have posted an Excel version in the File Library:

http://www.travellerrpg.com/CotI/Discuss/showthread.php?p=350267