12 Gauge Dummy Shell (insert & trim mandrel)

## Summary A 3D-printed insert that turns an empty, de-primed 12-gauge shotshell hull into an inert dummy shell. The insert has a spring-loaded plastic plunger, a snap-fit cap, and a high-contrast Dummy marking. The included trim mandrel helps produce a square cut at a nominal 56 mm from the hull base. The assembled dummy shell is nominally 60 mm long. The prototype shown in the photos was printed and tested. Fit can vary with the hull's internal profile and the printer's dimensional accuracy. ## Print settings - Printer used for the prototype — Anycubic Kobra S1 - Material — PETG - Nozzle — 0.4 mm - Layer height — 0.20 mm - Units — mm For two-color printing, select 03_Cap.stl and all five 04_Letter_*.stl files together and import them as one object with multiple parts. Assign a contrasting filament to the letters. The cap face and the letters share the same build plate coordinates. ## Parts and assembly You will need an empty, de-primed 12-gauge hull and a compression spring approximately 6.1 mm in outside diameter and 11.6 mm long in its free state. 1. Test-fit the Ø18.80 mm trim mandrel in the empty hull. Seat the mandrel, trim the hull flush with its front face, and remove the mandrel. 2. Fit the printed plunger into the guide. Place the spring behind the plunger and seat the snap-fit cap. 3. Fit the assembly into the trimmed hull. Check that the cap is secure and that the plunger moves freely and returns under spring pressure. Use only an empty, de-primed hull. Keep the finished dummy separate from live ammunition. Use a bright cap and contrasting lettering for easy identification. Long-term durability and compatibility across different firearms have not been established. ## Files | File | Part | |---|---| | 01_Guide.stl | Insert body, Ø19.05 mm in the model | | 02_Plunger.stl | Spring-loaded plastic plunger | | 03_Cap.stl | Snap-fit front cap, Ø20 mm in the model | | 04_Letter_1_D.stl through 04_Letter_5_y.stl | Five aligned letter bodies for the Dummy inlay | | 05_Trim_Mandrel_D18_80.stl | Hollow, removable trimming mandrel, Ø18.80 mm |