H&K platforms — particularly the MP5 family — are some of the most-modified firearms in the segment, and getting the fitment right matters. A3 catalogs every H&K-compatible product against the specific receiver type (MP5, MP5K/SP5K, UMP, or MR-series), with explicit notes on which length and mount style fits which variant.
For modular products like the HDA3 stock or our Modular Folding Brace, the firearm-adapter variant on the PDP locks compatibility at checkout — so the brace ships with the correct adapter for your specific H&K. Whether you're stocking out an SP5 for personal use, building a duty-grade UMP for an agency, or running an MR556 in a precision build, the lineup here is curated for fit, finish, and the kind of real-world reliability the H&K platform demands.
