45 ACP cartridge. Thompson’s Trench Broom hit quite a few snags in improvement, as neither a fuel-operated method nor a straight blowback proved suited to what he envisioned. What he wanted was a retarded blowback system. The initial A-five was built in the late 1990s. Discipline & Stream The Auto-five https://www.safeheavenfirearms.com/product/sigm400-tread/