The Common Gunnery Architecture (CGA) is a new software product line being developed by the Project Manager Combined Arms Tactical Trainers (PM CATT) at the Program Executive Office for Simulation, Training and Instrumentation (PEO STRI). The CGA objective is to reduce duplication between gunnery training systems in part by maximizing the advantage of common components such as the OneSAF Objective System (OOS). Using requirements derived from different gunnery training systems as a baseline, the CGA team analyzed the OOS Product Line Architecture Framework (PLAF) to determine OOS extensions required to support precision gunnery training. This analysis determined that the OOS PLAF would need to be extended or modified to include unique gunnery training components such as crewstation interfaces, image generators (IGs), curriculum and student management, simulation controllers and physical and behavioral models. By extending the OOS PLAF, CGA could heavily leverage the existing OOS infrastructure investment. As the CGA analysis progressed, the Research Development and Engineering Command (RDECOM) Simulation and Training Technology Center (STTC) was developing an embedded simulation demonstration using OOS to drive both the constructive simulation and virtual ownship simulation. This was closely related to the direction of CGA which encouraged the teams to leverage their efforts. As part of the risk mitigation for the CGA product line development, the CGA team partnered with the STTC to develop a functional prototype of a gunnery training system to verify assumptions concerning the use of OOS as the building block for the CGA product line. This paper provides an overview of the CGA program and identifies issues and lessons learned during the gunnery prototype development. Examples of the many issues include simulation engine latency, bi-directional interfaces and high fidelity models.