The ProMag magazines blend traditional steel stamping and welding with advanced precision injection molding using high-quality polymer compounds. Each magazine is covered by a lifetime guarantee against any manufacturer's defects. Specifically, the ProMag Sig Sauer P365-380 Drum Magazine features a construction from DuPont Zytel-based polymer, with a steel internal insert in the upper tower that is stamped and welded similar to the factory magazine design.