O.K., let's do some basic physics. There is a lot of really good information on the current Naval gun system: the Mk 45/54 Caliber 5 inch gun system at:
http://www.globalsecurity.org/military/systems/ship/systems/mk-45.htmThe muzzle energy of this system is 10-18 Megajoules (10 MJ for the Mk45/Mod 4, and 18 MJ for a more advanced unit with longer barrell and probably different propellant.) Project weight is probably somewhere in the neighborhood of 50-70 Kg (110 lb to 154 lb.) This gun system has a 'regular' range of 13 nautical miles, but with extended range ammunition (advanced rocket boosted, GPS guided munitions) this can be extended to 40 n.m.
Running some numbers for the Mk45:
Since KE=1/2*m*v^2 then rearranging to solve for "v" we get:
v=sqrt(2*KE/m)
For a projectile mass of 50 kg, muzzle energy of 18 MJ, we can expect to get about
v=848.5 m/s or about 2780 ft/s (a little slower than a 30-06 rifle bullet for you hunters out there!)
One of the number's 'tossed out' was Mach 5 projectile. Using this number as a WAG let's run some numbers.
Since Mach 5 at sea level is about 5*1043 ft/s=5215 ft/s. Let's use a muzzle velocity of 1 statute mile per second, 5280 ft/s, or 1600 m/s for metric buffs.
Let's look at a 20kg projectile of depleted uranium (this will be a dart about 5 cm in diameter with a 6 degree cone half angle and about 60 cm long (about 2 feet.) With a muzzle velocity of 1600 m/s, this project will have a muzzle energy of about:
KE=1/2*20kg*(1600m/s)^2
KE=25.6 Megajoules or about 70% more than the muzzle energy of the much heavier MK 45 shell.
This thing would definately pack a wallop! Incidently the "other" reason for choosing depleted urainum in armor defeating rounds has nothing to do with its density. It is that uranium metal is naturally pyrophoric--which means it burns spontaneously on contact with air when finely divided into powder. Well the impact of a highly energetic but very dense projectile will create jets of material that spalls of the penetrator as it flows through the steel armour. This jets of material will finely divide into microscopic droplets, which almost instantly burn in air. The net result is a catastrophic explosion inside whatever is penetrated all because of kinetic energy and pyrophoric chemicals. Inserting a pin of aluminum in the middle probably won't add too much to this, but I'm pretty sure it would burn just fine if it were a jacket.
Larger, faster projectiles are certainly possible and likely, especially if the Navy want's to go for guided munitions.
The problem is acceleration--rather too much of it. A rail gun projectile will accelerate pretty close to 100,000 g's which is about 10 times the g load of a conventional projectile: this means more difficult to engineer electronics with probably less capability. Longer barrels can help alleivate this, but that makes the weapon less compact. It's an interesting concept...