That's some nice gun pron
And it certainly got me thinking if I was off in scale by that much as you and other replies suggest.
So I decided to put some things in perspective.
http://i.imgur.com/rsjwqSI.jpg
As you can see, it makes little sense comparing ED guns to guns in fighter aircraft. An FDL is as long as a 747 and a lot more massive, and a size 4 gunmount looks oversized on an FDL.
I overlayed the Yamato gun turret and a 137mm deck gun (I thought it was a 200mm, but it isn't) on a Federal Corvette to show the difference. It;s mayube not quite a Yamato gun, but it's definetly battleship sized.
The issue that we run into by just looking at the visual size of the class 4 cannon without scaling up from smaller weapons is that the visual size and the performance of the weapon reach a point where they don't actually match up very well for the larger guns. A more accurate way of looking at the issue is based in part on visual size/length as a starting point for the smaller weapons but also looks at mass, projectile weight and projectile velocity. Weapon damage and effectiveness in Elite doesn't scale with length (i.e, size/volume), it scales directly with weapon weight, so essentially this means the largest Elite guns look "oversized" for their actual effectiveness. They're "battleship sized" only from a volume/length perspective, not a mass/damage perspective. Essentially the gun performance seen in-game falls within the range of 20 mm rounds (class 1 multicannon) to 200 mm rounds (class 4 cannon). Elite uses "mass" as a scaling factor, not length, which is why I've compared the class 4 autocannon to being equivalent to a 200 mm (i.e. 8-inch) naval gun.
If the only criteria is length and not mass or damage effectiveness, a class 4 cannon is "battleship sized" but it cannot deliver an equivalent damage output to an actual battleship gun. A single 16 inch cannon on an Iowa weighs approximately 120 tons and fires 1 ton shells. What we can do however is look at effective muzzle velocities and estimated projectile weight for the smaller Elite multicannons to determine what types of kinetic energy we're dealing with and scale up from there. I'm using stats for gimbal mounts in all my examples as they're what I currently have on my ships, you can make similar comparisons for fixed or turrets if you wanted.
Starting with a class 1 multicannon, it weighs 2 tons and has 2100 rounds of ammunition with a projectile speed of 1600 m/s. Let's assume that 1/4 of the total weapon weight is ammunition, i.e., 0.5 tons of ammunition,. Again, for argument's sake let's assume that half of the cartridge is projectile mass and the remainder is the casing and propellant which is comparable to most autocannon designs. That gives us 240 g cartridge weight firing 120 g projectiles with a muzzle velocity of 1600 m/s.
This actually compares quite well to a M61 Vulcan 20 mm cannon, which uses M50 rounds with a 260 g cartridge weight firing a 102 g projectile at 1030 m/s. The muzzle velocity of the class 1 multicannon is significantly higher, giving around 2.5-3X the kinetic energy of a 20 mm Vulcan round, but the rate of fire is significantly lower at only 8.3 rounds per second or approximately 500 rpm. A Vulcan fires at 6000 rpm, or 100 rounds per second, and in this sense a Vulcan at full rate of fire would probably do considerably more damage than the class 1 multicannon although we would expect fewer shots to be on target. Overall however we're dealing with a similar overall damage output in terms of roughly comparable projectile weights and muzzle velocities.
Going to the class 2 muticannon, making similar calculations, gives us 480 g cartridges firing 240 g projectiles, at a lower rate of fire of 7.3 shots per second or 440 rpm. The velocity for all multicannons remains the same at 1600 m/s, which basically means we have double the projectile weight at 7.3 rounds per second instead of 8.3 rounds per second. This matches up quite well with the in-game damage ratings for the class 1 and class 2 multicannon, which have double the damage from 0.8 per round to 1.6 per round, which tells us that FD is also scaling gun damage based on total weight and assumes that projectile mass doubles with other parameters staying the same. In other words a class 2 multicannon fires projectiles that are double the mass but travel with the same velocity and a slightly slower ROF. This compares well to the projectile weight for the 30 mm cannon rounds fired by the GAU-8, which have a weight of around 380 g and a similar muzzle velocity to the M61 Vulcan's 20 mm rounds.
You can do similar calculations for class 3 and class 4 multicannons, making their calibres equivalent to approximately 40 mm or 50 mm rounds. Anything larger would need to be more than double the projectile mass with each increase in weight and we know that the projectile mass should approximately double with each increase in size category. Here's a good visual comparison of the difference between 20 mm Vulcan rounds and 30 mm GAU-8 Avenger rounds, you can extrapolate that up to 40 mm and 50 mm rounds and that is about the limit for the larger multicannons in Elite if we are limited by doubling the projectile weight with each increase in multicannon class:
Now let's take a look at the class 4 cannon. It weighs 16 tons and has 100 rounds of ammunition. Let's assume again that 1/4 of the weapon weight is ammunition, i.e., 4 tons of ammunition, again assuming a 50% weight for projectile vs. casing/propellant. That gives us projectiles that weigh 20 kg each. Unlike multicannons the projectile weight varies with different types of cannon mounting, but for a gimballed class 4 it is 750 m/s. That compares reasonably well to the 200 mm (8-inch) naval guns, with the BL Mk VIII weighing 17.5 tons and having a similar muzzle velocity of around 850 m/s although they fire a significantly heavier round weighing 116 kg. In that sense we start to see the significant damage dropoff where Elite cannons don't really scale up in terms of damage potential anywhere near as well as they should based on their visual size. If anything we're seeing substantially lower powered cannons despite their impressive visual size, with a class 4 cannon looking the same size as a much larger naval gun but firing projectiles comparable to a visually much smaller weapon.
There is nothing you can mount on any of the player-operated ships that actually rivals a 16-inch battleship gun that weighs 120 tons and fires 1 ton shells at around 800 m/s. Even if the class 4 cannons in Elite were capable of doing this with extremely efficient propellant they would end up with an ammunition capacity of only 4 shots, not 100, and that leaves us with only 12 tons for the gun itself, which would be only be 1/10 the weight of a battleship gun. Essentially the class 4 cannon might look like a battleship gun but it only weighs 1/10 of the weight and fires projectiles that are only 1/10 of the mass of an actual 16 inch battleship gun.
Now, you could argue that Elite uses materials that are quite a bit stronger for their weight than modern steel and ship durability in Elite is certainly much higher than what we could produce with modern armor. In terms of kinetic energy however we have known maximums for projectile speed (which lists velocities in the outfitting screen) and we know the maximum ammunition weight that would be reasonable for the weapons to carry (which we are assuming are roughly 1/4 of the weapon emplacement weight that would likely consist of ammunition storage). You could also argue that a 200 mm cannon projectile in Elite uses superior explosives compared to what we can produce with modern technology, and in that sense they might inflict more damage than modern weapons, but the guns themselves are still not firing projectiles that have weights and velocities that rival a 16 inch battleship gun. The guns on Elite ships might LOOK battleship sized when you get to the class 4 cannon, but they are not firing projectiles that have an equivalent projectile mass and muzzle velocities of a battleship gun. At most you could consider the class 4 cannon equivalent in performance to a 200 mm naval gun (i.e., 8-inch naval gun) found on WWII-era heavy cruisers, and even then we are seeing a significant dropoff in projectile size and weight with a much smaller round being propelled at a similar muzzle velocity. If you want true battleship-level cannons you're going to need to go several steps up from that in terms of gun and projectile weight, i.e., you need to increase the projectile weight by approximately a factor of 10 in order to reach the level of firepower seen from a 16-inch naval gun.
The cannon batteries on the Farragut-class would certainly qualify for these types of sizes but they are considerably larger than a class 4 mount. Using the rule of doubling the weapon and projectile size for each increase in size category, a class 5-6 weapon would be equivalent in weight to a 11-12 inch naval gun at around 32-64 tons total weight, i.e., a weight of 24-48 tons for the gun with 8-16 tons of ammunition. The equivalent to a 16-inch battleship gun in Elite would weigh approximately 96-192 tons, i.e., a weight of 72-144 tons for the gun with 24-48 tons of ammunition, which would be considered a class 7 or 8 weapon mount in Elite outfitting terms.
