[Somehow gmail separated this post out into my "Promotions" folder, so I almost missed it . . . ]

How about applying the squeezed/tapered bore principle?

From wikipedia:  

"Armour-piercing, composite non-rigid

Armour-piercing, composite non-rigid (APCNR), the British term, but the more common terms are squeeze-bore and tapered bore and are based on the same projectile design as the APCR - a high density core within a shell of soft iron or other alloy, but it is fired by a gun with a tapered barrel, either a taper in a fixed barrel or a final added section. The projectile is initially full-bore, but the outer shell is deformed as it passes through the taper. Flanges or studs are swaged down in the tapered section, so that as it leaves the muzzle the projectile has a smaller overall cross-section.

This gives it better flight characteristics with a higher sectional density and the projectile retains velocity better at longer ranges than an undeformed shell of the same weight. As with the APCR the kinetic energy of the round is concentrated at the core on impact. The initial velocity of the round is greatly increased by the decrease of barrel cross-sectional area toward the muzzle, resulting in a commensurate increase in velocity of the expanding propellant gases."

In the real world, this type of barrel was superceded by the development of APDS ammo. APDS gave you the same small-bore heavy-core effect, by letting a lighter sabot drop away to expose a fin-stabilised tail. This allowed you to use normal rather than tapered barrels. But in the vacuum of space you can't fin-stabilize rounds, so you need actual tapered bores to get the effect.

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