Margraf, R.A. Halavanau, A. Huang, Z. Krzywiński, J. Li, K. MacArthur, J.P. Marcus, G. Osaka, T. Robles, R. Sakdinawat, A. Sato, T. Sun, Y. Tamasaku, K. Zhu, D. JACoW Publishing Geneva, Switzerland Bragg-reflecting cavities on the 10 s or 100 s of meter scale are a core component of proposed Cavity-Based X-ray Free-Electron Lasers (CBXFELs). While CBXFELs promise improved longitudinal coherence and spectral brightness over single-pass self-amplification of spontaneous radiation (SASE) FELs, construction and alignment of large Bragg-reflecting cavities can be difficult technical challenge. Our collaboration recently demonstrated stable operation of a low-loss 14 m 9.831 keV X-ray cavity of four Bragg-reflecting diamond mirrors*, a significant step towards a CBXFEL-scale cavity. We in-coupled X-rays from the Linac Coherent Light Source (LCLS) into our cavity via a transmission grating, then measured round-trip efficiencies approaching 88%, or >96% when neglecting losses on in-coupling and focusing optics. Additionally, we characterized transverse oscillations in the cavity, demonstrating the effectiveness of our cavity focusing. We will discuss these results, additional new analysis and consider implications for future CBXFEL projects.