I highly doubt that anyone who 3d prints a lower does so to “use” it (I.e. shoot someone) in order to procure a better firearm.

The FGC-9 was used extensively in Myanmar for that exact purpose. The rebels would set up ambushes with FGC-9's and recover better firearms for future use

https://en.wikipedia.org/wiki/FGC-9#Users_and_use

I would think they just print multiple guns and switch if one breaks.