German WWII indoor training cartridge fired through a sub-caliber insert installed directly inside a standard service pistol. Allowed soldiers to practice marksmanship indoors using their actual duty weapon without live ammunition logistics.
Type: Pistol
Introduced: 1940, Germany
Parent case: None
Standardization: Obsolete / Non-standard
Bullet diameter: 0.157" (4 mm)
Case length: 0.492" (12.5 mm)
Overall length: 0.748" (19 mm)
Max pressure: 12,000 PSI
Rim type: Rimless
Primer: Small Pistol
Typical twist rate: N/A
Muzzle velocity: 450–550 fps
Muzzle energy: 3–4 ft-lbs
Effective range: 15 yd
Common bullet weights: 6 gr
Primary use: Training
Production status: Obsolete
Also known as: 4mm M20 · 4mm Übungs · 4mm Training
History: The Wehrmacht faced a training ammunition shortage as WWII progressed — live 9mm Parabellum was too valuable to burn on stationary target practice, and dedicated training ranges were being destroyed by Allied bombing. German ordnance engineers developed a steel insert that fit inside the barrel and chamber of the standard Walther P38 and Luger P08, reducing the bore to 4mm. Soldiers loaded the…
Notable firearms: Walther P38 (with insert), Luger P08 (with insert), Various WWII German service pistols
Military use: {"country":"Nazi Germany","years":"1940-1945","conflicts":["WWII"]}
Similar cartridges: .22 Short