Even in one shot the 3d printed gun will explode. The cartridge is just a container for the gun powder, not the explosion. Real guns have a chamber that contains this explosive pressure.
3d printed guns are nowhere near strong enough to contain this pressure and when the gun fires the bullet is flung harmlessly in some random direction. Since there is almost no energy imparted into the bullet it doesn’t have any power or lethality, heck the shrapnel from the casing is literally more deadly for the shooter than any bullet towards the shootee.
Heck a 3d printed gun can even fire a bullet at all. Plastic is not rigid enough to detonate the primer and set the round off. You can literally fry bullets in a cheap metal pot and when they explode they won’t even go through the pot.
The only way you could make a 3d printed gun work is by incorporating tons of other metal parts, at which point it isn’t a “3d printed gun”. Search up pipe shotguns. They can be made with a handful parts from home Depot and only require 1 or 2 tools at home (only 1 if you get them cut at home Depot). Far more effective and actually deadly, even used by guerilla forces against imperial Japan in the Philippines.
In all seriousness improvised ammunition isn’t very hard and it’s doable. All you really need is primer powder and some kind of explosive. I won’t say how but both can be easily made and are pretty readily available. In the rare cases of id required, you can just start asking friends and neighbors if they have any handy.