Reserved for Energy Guns
Simple Recommendation: Base them off of bows / crossbows. Powered by energy gems rather then arrows / bolts. Each Energy Gem is good for 20 shots and then is discharged. Energy Guns deal Energy Damage based on the energy gem used to power them.
Perhaps there are five types of Energy Gem (For each energy type [discounting Force]), priced appropriately or price gouged appropriately. Go to the frozen north, ask for a Fire Crystal. Get laughed at.
More thoughts: Energy Gems can be recharged, but cost the same as buying a new one. They're cheap because the enchantment on them is incredibly specialized, and the gems used are essentially useless cast offs from Crystal Mages using the rest of the gem for an actual effect.
Standard and Enhanced Energy Gems: Standard energy gems deal energy damage. Enhanced Energy Gems either deal more damage OR have additional effects as per the
Orb of... spells from Complete Arcana (Acid sickens, Cold blinds, Electricity entangles, Sound deafens, and Fire (SHOULD) set things on fire.)