Looks like a lego guy at first.

The significant plumes of smoke generated when the weapon was fired indicate that it does not make use of commercial small arms ammunition propellant (‘smokeless powder’), and may instead use blackpowder or an alternative propellant. This makes the use of ‘separate-loading’ ammunition (i.e., propellant and projectile loaded separately into the weapon) more likely, as well as increasing the likelihood that the weapon was a muzzle-loading design—that is, loaded from the bore (‘front’ of the barrel), rather than the breech (‘rear’ of the barrel) of the firearm.

the weapon appears to use an electric firing mechanism. Images of the firearm show that an electrical wire passes through each endcap. …This high voltage creates a hot plasma arc between two conductive contacts that can be used to ignite flammable materials

So this is where Bungie gets their exotic weapon designs from for Destiny

Probably the most prominent electric firing mechanism for 3D-printed firearms has been developed by the user ‘@SuckBoyTony1’. This mechanism uses an 80 kV High Voltage Pulse Generator that converts 6–12 V (the electric potential typically provided by battery packs such as that seen with the assailant’s weapon) into 80 kV. This high voltage creates a hot plasma arc between two conductive contacts that can be used to ignite flammable materials

What’s with this level of overengineering? Just get the Piezo from a lighter and maybe that igniter powder from fireworks and good.