Hello,
NOTE: What I present here is VERY simplified (yet STILL very long!) Ballistics is a BIG topic and many details have been left out.
The projectile is larger than the standard projectile used in the AR-80 assault rifle, and at supersonic speeds
That would mean tremendous recoil. Perhaps make the projectile smaller than the assault rifle ammo?
Railguns do not produce any recoil, this is what made them so attractive for the Starwars programme.
With a black-powder weapon, you have an explosion, which moves in all directions. Because the combined mass of the weapon and the user is greater than the mass of the bullet, the bullet is pushed out of the end of the weapon. If the mass of the bullet were the same as the mass of the bullet, the weapon would move just as far & as fast as the bullet. (Newton's 3rd Law)
The reason humans lose accuracy when affected by recoil, is actually due to muscle spasm; when the force of the explosion pushes the rifle back, the energy is transfered into the arm (via the wrist). This causes involutary contraction of the muscles in the arm, which move in their natural directions (i.e., upwards). This effect can be exaserbated by the design of the weapon (pistols and subs move the bolt back, which affects the centre of balance; lookup the design of Uzi 9mm for more info...)
With the railgun, the energy is generated by a magnetic field, with the 'action' force action along the length of the weapon, with the 'reaction' forces being directed outwards, rather than backwards (thus maintaining Newtons 3rd Law). Because the forces go out to the side, with equal magnitude, the weapon stays still. The only problem (as pointed out on Wikipedia) is that the barrel of the weapon could be pushed appart (but that doesn't happen because it's properly designed...).
One way of thinking about it, is that the slug is "pulled" out of the front, rather than pushed.
Also, railguns are traditionally touted as hypersonic (mach 7.5+). The fastest rifle bullets travel at about mach 3. You gain effectively twice the range and power, or better. In theory, one could construct a 'small' railgun that was just as fast as a normal rifle. The only advantage this would offer, is that there would be no recoil, which would make it absolutely devastating for burst fire (a burst is just as good as an aimed shot).
it is capable of doing significant damage to any soft target unfortunate enough to get in the way
From what I understand, damage is huge against tanks and body amour (BTW, is there a way to specify that in UFO AI? What is the meaning of the second number in the damage field?) but small agains soft tissue (especially if the projective is light).
Damage is all to do with energy transfer. If you have a railgun that fires a slug the same size & shape and speed as a normal rifle, it will do about the same damage. (The traditional lack of rifling on a railgun would alter the characteristics slightly).
The reason lead is used, is because it is very dense (amongst other reasons). This means, that the mass for the size of the shot is maximized. As energy transfer is effected by the mass of the two bodies involved, having a heavier slug is better. That said, lead is relatively soft and tends to deform (squish) when it hits something hard. Depleted uranium is EXTREMELY dense and harder than lead, which is why it is so popular for armour piercing ammunition. Steel and tungsten are also used for armour penetration, as they are very hard and tend to keep their shape better.
The key to considering 'damage' is to understand how the energy affects the target. Because 5.56mm rounds are so small, they affect a very small surface area, this results in very little energy being transfered to the target, which is why they often shoot straight through people, without knocking them over. The bigger 7.62mm slug hits more flesh, which means more energy transfer.
For example if a 5.56mm bullet and a 7.62mm bullet have the same energy (mass x velocity), then the 7.62mm would do more damage to soft tissue. However, if one were trying to shoot through something (e.g. armour), 5.56mm would be a better choice, because the smaller size means less area for the armour to catch, which means less energy will be transfered to the armour.
Of course, in reality, 7.62mm slugs have more energy, which makes them better at penetrating armour and cause EVEN MORE soft tissue damage.
Your point about the ammunition being caseless is correct; One could carry just as many slugs that were the same size as a 5.56mm
cartridge, as would could 5.56mm cartridges. If one just wanted slugs that were ths same size as a 5.56mm
bullet, then one could carry many many more.
As for noise, well, railguns do not have an explosion that sets off the projectile, making them much quieter. The supersonic round would make noise... after it passed / hit the target. As to whether this would qualify for 'suppressed' equivalent opperation, I'm not sure. On the otherhand, a hypersonic railgun will cause contrails a-la Quake 2 and "Eraser" (these are dramatisations of a genuine phenomenon), including these would remove the stealthiness.
In summary, the best way to describe the railgun, is to design weapon stats that make a
fun game. Then, based on the following, characterise the weapon.
1. If it is powerful against flesh, it has a big or flat slug
or it fires lots of small slugs in pre-programmed bursts (that in game terms are just one shot)
or it fires a "flechette"/shotgun style round.
2. If it is good at penetrating armour, it has a large(r) dense, shaped slug
or fires an 'ordinary' shaped slug very, very fast (hypersonic) see also: Sabot.
3. If it has lots of ammo, it has smaller slugs.
4. If it has normal amounts of ammo, the ammo is very light compared to a rifle (1/3 to 1/2 the normal weight).
My instinct is for a hypersonic small calibre rifle (5mm ish) that fires pre-set bursts of shaped AP slugs as an standard weapon, with a heavy (12mm/.50cal) single-shot as a man-portable anti-tank weapon.
