According to Wikipedia, Polylactic acid, a.k.a. PLA “is soluble in chlorinated solvents, hot benzene, tetrahydrofuran, and dioxane” and is not soluble in acetone, alcohol, or water.
Just to see what happens, I dunked a pair of those 3D printed dummy bullets in Shooter’s Choice Gun Solvent (which has since gone obsolete) and Hoppe’s No. 9 Gun Bore Cleaner (which seems to have been reformulated several times), then let them air-dry in those background puddles:
Nothing much happened: they’re not soft or gummy, haven’t slumped, and seem undaunted.
That’s in contrast to ABS plastic, which is readily soluble in acetone and the aromatic hydrocarbons commonly found in solvents used around firearms. Apart from that, ABS would be a slightly better choice on mechanical grounds. I’m not sure the difference really matters for most purposes, given the very wide tolerances on 3D printed objects.
The Smell of Molten Projects in the Morning 
Have you ever made Red’s gun cleaner? http://home.comcast.net/~dsmjd/tux/dsmjd/tech/eds_red.htm
Zowie! Sounds like good stuff. [grin]
At the rate I use it, though, I may have just slightly less than a lifetime supply of the commercial cleaners. The quantities in that recipe would produce an heirloom to be passed down through the generations…