Tidal Locking plus Zombies
Use caseless rounds, electrical ignition. Very compact and reliable, with minimal maintenance needed, and can fire reliably in most any temperature. Common ammo types.
Bullet with an explosive substance at the back end. Ignited using electricity. The most standard round is made of a soft metal, such as lead. Gunpowder is versatile because there are a number of bullet designs and materials for any situation.
Don’t need recharging batteries. electronics are hardy enough that they haren’t easily affected by EMPs. Can rapid fire.