What about removable cell? You still have recharges but if you want to, with for example proper access to balance it a bit, you could remove basic powercell from it and swap it with charged slime core or bluespace one, making a normal energy gun a much more dangerous spammy lategame weapon?
Bonus points - more interactions between sec and science
Alternative idea - if all energy gun get selfrecharging, that means they get wireless transmission of energy which comes from... Engine! What if, depending on power output, energy weapons could get increases charging rate or even damage! That will make a reason to set up a supermatter engine (tesla could be safe and easy power source, but lord singuloth would produce much more power to the sec team) and also a reason for antags to sabotage engine (in Non-destructive manner ofc)