But the tooling required for guns and rifling in particular is pretty hard to come by, whereas the tooling required to assemble a DIY drone is basically indistinguishable from R/C hobby equipment. You can (try) to regulate where/who/how/when can fly a drone, but regulating drone building seems almost impossible to me.

The tooling for guns and rifles is not hard to come by. The most difficult part of making firearms from scratch is the primer in the ammunition.