Unlike most modern electronic weapons, the functionality of fully-mechanical rifles are complemented by their durability. They can be used without authorization, electricity, or calamity-proofing.
However, plasma weapons are a different topic.
Gas magazines are easily contained, or harvested. You just need a pressurized tank with a trigger-controlled valve.
But, ionizing them into plasma, and then firing that plasma with electromagnets, more or less requires electricity, and electricity requires a battery, or at least an energy buffer, to be portable.
Everything that works with electricity is in danger of not working properly in presence of an EMP, lack of insulation, or lack of power source.
So, here goes.
What mechanism should a plasma rifle have for it to still work when there's a storm, when underwater, or when an EMP bomb is detonated?