Production began in late 1944 and they were on issue by the March 1945 Operation Varsity airborne crossing of the Rhine into Germany. Inglis produced two versions of the Hi-Power, one with an adjustable rear sight and detachable shoulder stock (primarily for a Nationalist Chinese contract) and one with a fixed rear sight. The plans were sent from the FN factory to the UK when it became clear the Belgian plant would fall into German hands, enabling the Inglis factory to be tooled up for Hi-Power production for Allied use. High-Power pistols were also produced in Canada for Allied use, by John Inglis and Company in Toronto. The cam slot and bar move the chamber upward and the locking lugs on the barrel re-engage those in the slide. This also pushes the chamber and barrel forward. After the slide reaches the limit of its travel, the recoil spring brings it forward again, stripping a new round from the magazine and pushing it into the chamber. The downward movement of the barrel disengages it from the slide, which continues rearward, extracting the spent case from the chamber and ejecting it while also re-cocking the hammer. The barrel and slide recoil together for a short distance but, as the slot engages the bar, the chamber and the rear of the barrel are drawn downward and stopped.

Unlike Browning's earlier Colt M1911 pistol, the barrel is not moved vertically by a toggling link, but instead by a hardened bar which crosses the frame under the barrel and contacts a slot under the chamber, at the rearmost part of the barrel. The Hi-Power, like many other Browning designs, operates on the short-recoil principle, where the barrel and slide initially recoil together until the barrel is unlocked from the slide by a cam arrangement.

Fn hi power serial numbers 225xx