3D printed guns have had one major drawback- the materials they’re made from don’t hold up well to the stresses created by an exploding bullet. The plastic cracks and the barrel or receiver becomes useless quickly. One developer has found a way around this by machining thick walled shells with the bullet an inch or so inside them, which act as little barrels to contain the explosive force. They have to be machined individually, but the raw material is cheap and they can be reloaded after use.
Bullet could make 3D-printed guns practical deadly weapons (Wired UK).