Add to that the fact that some gun parts are not printable and have to be made out of metal. Printing an entire firearm on a $500 3D printer is possible but that gun will be good enough for 1 bullet, and probably will hurt shooter at that.

Also, if inmates can make a handgun out of whatever they have access at prison - 3d printing is the afterthought. Some parts can be bought in a department store, the others can be ordered online as a replacement parts. If someone really wants to make a gun themselves, 3D printing ban wont stop them.