I've put that gun to the locomotive's equipment grid, then entered into it (manual mode) and got the crash.
Locomotive does not have inventory. Could you check the player's inventory in this case?
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The mod Gun Equipment (0.0.16) caused a non-recoverable error.
Please report this error to the mod author.

Error while running event GunEquipment::on_tick (ID 0)
GunEquipment/AutoGun/control.lua:28: attempt to index local 'ammoInv' (a nil value)
stack traceback:
GunEquipment/AutoGun/control.lua:28: in function 'ReloadUnloaded'
GunEquipment/AutoGun/control.lua:108: in function 'Reload'
GunEquipment/AutoGun/control.lua:215: in function 'OnTick'
GunEquipment/control.lua:20: in function <GunEquipment/control.lua:13>

Both common and nuclear locomotives don't have inventory (at least in vanilla and RampantArsenal) and this mod isn't ready for this situation. Vehicle's inventory is essential.

at least it can do not work, but it should not crash.

I'm sorry for leaving this mode for long time without reactions.
And yes this may be fixed in future, but for now please do not use entity without inventory.

playing with local patches...