Actually, in theory, a bullet(so long as it didn't use a combustion-based firing agent, hydrogen/oxygen mix, or even h-fuel, would suffice) would carry the same speed as when it left the barrel, and as I am about to reveal...
extracted from
http://www.probertencyclopaedia.com/F5B.HTM
"The M14 is a US automatic rifle developed in the 1950s to replace the Garand. It takes a 7.62 mm round from a 20-round box. It has a muzzle velocity of 853 meters-per-second and is sighted to 915m with a cyclic rate of 750 rounds-per-minute."
If the M14, an outdated US rifle, which is already surpassed by four models(the latest being the M4 tactical rifle)... you think the muzzle velocity would be HIGHER than those energy guns. My idea is this... a machine gun. Complete with ammunition. and would go much further. My suggestion for counterbalance is that the shield damage be absolutely minimal(shields were designed to take large rocks flying at you, why shouldn't they stop bullets), but should have above-average damage to the hull. Well above average.