[JoeFromSidney]

It's possible to make magazines for existing guns by 3D printing. However, the next step will be printing the guns. However, there will be redesign of the guns. First, the printed barrel must be wound with steel wire or fiberglass, to provide strength. Instead of using gunpowder, the gun will be an airgun, like those used by the Lewis & Clark expedition in the early 1800s. Their airguns were capable of putting a bullet through a small tree. Make the barrel smoothbore, and make the projectile a rifled slug. Probably it won't have the accuracy of a good sniper rifle, but it should be adequate for most purposes. The air pressure reservoir will have to be patterned after the high -pressure, fiberglass-wound air tanks used in missiles. The problem will be re-pressurizing in the field. It obviously can be done. Lewis & Clark managed it. Anyway, I think this is one way to go with a completely 3D printed gun.

[ArrogantBustard]

Make the barrel smoothbore, and make the projectile a rifled slug.

A fin-stabilized projectile with a discarding sabot would be interesting also.