Luigi Mangione

The defense has access to that evidence and can analyze it. When taken apart, a weapon that was printed in a non-functional state will be different from one that broke during use. Being non-functional might have missing parts or bad fitment or such. A broken 3D printed firearm would have breaks/delamination that tend to be along layer lines. The defense can call an expert to testify and tear the whole case apart.

Only dumb cops would try that. Like the cops in a small rural PA town. We already know they're not that smart because they've likely fucked up the chain of evidence. Except, that alone is enough to sink the case, and we don't need to assume anything about how the gun got there.