simple dimple flare gun bore conversion is the simplest and one of the more cost-effective ways to put a dimple in your target. the goal of this was to make sure a gun could find its way into the hands of any man whether you have years of experience or none at all with 3d printing all that is required for this specific iteration is a rifled insert (available in the links below) a 3/8 drill bit with a drill and epoxy or any other strong adhesive. 

this has been successfully test-fired in PLA, printed on an ender 3 at 50% infill and .2mm layer height without the use of a soldering iron for layer strengthening. while it did hold up fine test any lower strength configuration at your own risk

this was test-fired in an Orion 25mm/12gauge aquatic flare gun and requires NO modification for full function and can be swapped back to flares as easily as removing the bore conversion

rifled barrel insert:

https://www.ebay.com/itm/123976953813?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2060353.m2749.l2649

Orion flare gun:

https://www.amazon.com/Orion-Safety-Alerter-Launcher-Aerial/dp/B07XM4FZ37

working on a centerfire one but no promises.