BTW folks, there are some technological issues with railguns that have not been discussed here yet. Among others is the fact that railguns produce their own EMP event. That means the ship's general electronics will have to be well-shielded from the EMP every launch produces. I only mention this because this increases the cost of railgun installation and use.... and putting one on an existing naval platform is unlikely. As one of you mentioned earlier... the platform will probably be built for and around the railgun itself. The shielding issue by itself would probably require this. And... let's talk "real world" here. That shielding probably needs to be around much of the ship's general spaces, because that same EMP burst could burn out personal electronics devices of the crew... laptops, tablets, cell phones, etc. Anything with a chip. Shielding the railgun itself isn't useful because the EMP is part of the "muzzle flash" so to speak.
The EMP from the muzzle is relatively minor because the arc and magnetic fields will not extend much beyond the rails. The shielding on the gun is supplied by the structure holding it together (when firing the rails repel each other). A simple structure like a muzzle break could block the rest, unless you are directly in front of the barrel (and therefore have more important concerns)
The shielding you want for the rest of the vessel is a Faraday Cage, and is already built into the vessels structure for EMCON purposes. What’s the point of locking down all your systems if someone off duty composing a message to his family on his iPhone can give you away.
Second interesting issue no one has mentioned: it is highly useful, and therefore very likely, that any production railgun would include a mechanism for what has been called "pre-injection." This is accelerating the projectile to Mach 1 or better BEFORE THE PROJECTILE ENTERS THE RAILGUN ITSELF. Sorry for the capital letters, but needed to make the point. Using the magnetics to launch a projectile from rest is extremely energy consumptive, leads to extensive ablation of the rails and other metal parts, and can result in tack-welding of the projectile or its sabot to the rails. That event would probably cause what the space industry called a "catastrophic failure," fancy jargon for lots of fireworks and crew deaths,
By injecting or propelling a projectile to Mach1 speeds as it enters the breech of the rail section allows more efficient use of propelling electrical energy, avoids the tackweld problem, and reduces the wear-and-tear on the rails, projectile, and associated element. The most efficient way to do this probably would be using compressed gas. Nitrogen comes to mind, though compressed air might be a cheaper alternative if workable. (Steam could accomplish the same thing... but water would play all heck with the railgun part.)
That is 1980’s technology, where you need to build a cannon on the end of a cannon, it is just too big to be practical. They do it now using pulse shaping technology, something that was not available back then (it is an outgrowth of variable speed DC motor R&D, which in turn was a byproduct of developments to deal with a major electric vehicle design issue). Basically you start out at low power, then increase the power level as the projectile accelerates. This greatly reduces the tendency for
tack-welding as well as reducing the cooling and erosion issues with the rails. But not to the extent that they can claim to have solved those problems.
The technology used back then for accelerating the projectiles is called a
light gas cannon, a 2 stage device that used a piston in a cannon barrel instead of a projectile to compress helium or hydrogen gas until a predetermined pressure was reach, at which point either a rupture disk opened releasing the gas into the main barrel, or a shear pin breaks releasing the projectile, and the gas propels the projectile down the barrel. Why use such a complicated arrangement? Because the average velocity molecules in a gas is proportional the square root of the temperature divided by the molecular weight. The main components of the propellant gases in a conventional cannon is carbon monoxide and nitrogen oxide (mw=28 and 30), so hydrogen gas (mw=2) moves about 3.8x faster, so the pressure in the barrel drops slower. What this boils down to is if the peak acceleration is that, compared to a high velocity cannon of the same barrel length, a light gas cannon, the average acceleration for the light gas cannon is something like 2x higher, with 50% increase in muzzle velocity, and even more is possible because you can use longer barrels that for the conventional cannon.
Third thought..... the projectile probably will sit in front of an armature, a conductive element that is part of the entire physics of railguns. Think of the armature as a sort of sabot actually propelling the projectile, rather than the projectile itself being part of the whole conductive thing. The projectile itself might be some sort of carbon material with embedded conductive metals like oxygen-free copper. Carbon has lubricating properties that could reduce rail damage to allow a greater number of launches before rail replacement.
Railgun projectiles either have to be non-conducting or carried in a non-conducting sabot. The armature is as you noted on the back, but in many designs it is little more than a strip of metal that vaporizes to create an electric arc that functions as the armature. The projectiles will probably be tungsten or depleted uranium for maximum density and energy retention, and are likely to have a ceramic coating to reduce heat transfer from passing through the atmosphere, i.e. much like an APDSFS, except with a skirt instead of fins that would burn off too quickly at those velocities.
