Finish Your FMP21/HK21 Build — HK21 Reinforcement Rails
The HK21 platform requires welded reinforcement rails in the receiver — and we’ve got you covered. STS has sourced original G3 stock rails from Germany and, using a custom-built fixture to handle their tapered, teardrop shape, we’ve precision-modified them into proper HK21 reinforcement rails.

These are genuine HK factory G3 rails, transformed in-house to meet HK21 specs.

• Material: German HK Steel

• Includes: One left and one right rail (sold as a pair)

• Fitment Note: Due to external variations between HK21 receivers and variants, minor fitting may be required.

These rails are an ideal solution to move your HK21 or FMP21 build toward completion with authentic, properly-shaped reinforcement components.