Ballistic coefficient is a function of mass over diameter. Bigger bullets are more efficient. For example, the 69 grain Sierra MatchKing bullet for the 5.56mm has a BC around .305. The 200 grain .338 bullet BC is around .750. I’m told they are tremendous fun in the .338 Lapua. The 16” round is actually very efficient.
The Copperhead has a muzzle velocity around 1700 f.p.s. from a barrel about 20 feet long. I exaggerated when I said the rail gun might fight on the Nimitz, which is just over 1,000 feet long. In order to make it possible to aim the thing and install it on a ship, the barrel would be more like 50 feet long. You can see the difference. The Copperhead accelerates at around 4,500g. The rail gun, if we use a 50 foot launch platform, accelerates to 10,000 f.p.s. in .005 seconds. This is 62,000g.
Also, the Copperhead cost $40,000 in 1998. It has a 30 pound warhead. I expect that a special ship would have to be designed to deploy the rail gun. It would be a nuke, so power is not an issue. But it would have a crew and maintenance, so the cost would must include capital outlay and service. Meanwhile, we already have planes that can deliver weapons with similar destructive power and we have guys who get off on flying them and taking pictures of Achmed as he gets smoked. I rather enjoy those videos.
I just don’t see the need for this thing.
And how much do those planes cost, and for the Navy what about the $6 billion runway?
I generally don't like conversations like this because people tend to support their favorite platform. The truth is that all the systems should be complementary. You won't use a rail gun against troop formations, you need a cluster bomb or similarly loaded Tomahawk for that. You won't use this against an underground bunker, as we have bombs just for that. But if you've got a ship over the horizon, it's pretty cheap to send a few of these downrange instead of a Tomahawk, and they'll hit the target sooner than any other system in use today (like maybe before they launch their missiles).
That is unless we can bring back the Nike Sprint repurposed for surface-to-surface. Those were super-cool. Designed in the 60s, they had active homing radar and accelerated at 100g (15 seconds from launch to an altitude of 30,000m).