This is a suppressor for the Crosman 2240 (or similar). This only fits airguns with a .4375 diameter barrel and a flat on one side. This suppressor is for use on airguns only.
This should print without supports when stood on end (Airgun Only text down).
The hole is printed with threads for a 10-24 3/8" length set screw. You should probably run a tap through the hole about 90% of the way (leave the last 10% to keep the screw in tight without backing out).