This model set turns an Action Army AAP-01 into a M6G from Halo. The pieces attach to the AAP-01 with only a couple of bolts and no modifications to the pistol itself are required (though you can cut the inner barrel short to be flush with the front). The handguard slides and snaps into place around the grip for a secure fit.
In addition to the printed pieces you will need one M5x10mm bolt and one M3x12mm bolt. The two handguard pieces should be glued or welded together.
This set includes seven models in .stl format, and instructions on how to assemble everything.
Don’t have a 3D printer? A set of prints and hardware are available here.