The FN Model D was the most fully developed variant of the venerable Browning Automatic Rifle. Introduced in 1932 by Fabrique Nationale (with whom Browning had collaborated extensively prior to his death in 1926), the FN-D had a number of significant improvements over the standard BAR. It used a pistol grip instead of a traditional stock, a shorter quick-change barrel, improved rate-reducing mechanism, and simplified disassembly procedure. The locking system of the gun and general receiver design remained identical to the standard BAR.
One of the main complaints against the BAR in combat was its fixed barrel. The concept of an automatic rifle for walking fire was obsolete virtually as soon as it was introduced, and the BAR in combat tended to be used as a light machine gun. The fixed barrel was easy to overheat, and American armorers were regularly replacing destroyed barrels on front-line BARs. The FN-D addressed this issue by finning the barrel for better heat dispersion and allowing it to be easily changed. In addition, the FN-D mounted its bipod on the end of the gas cylinder instead of the barrel itself. This reduced the weight and bulk of spare barrels (a lesson that would have been wise for the later M60 designers to take note of).
The FN model D continued the use of the fire control mechanism from the original BAR, with “slow” and “fast” (350 and 600 rounds per minute, respectively) instead of a semiauto and fullauto selector. They were also made in two receiver types, one for .30-06 caliber and one about 1/4 inch shorter for shorter cartridges (primarily the 7×57, 8×57, and 7.62 NATO). The 7.62 NATO variant was designated the FN-DA1, and used the same magazine as FN’s FAL rifle. In addition to factory-made conversions for 7.62mm, the Israeli military also converted many 8mm FN-D machine guns to 7.62 NATO, and these kits can still occasionally be found in the US. Magazines for the 7×57 and 8×57 guns are identical, and both they and the slightly longer .30-06 magazines are distinguished by the large “X” stamped in them.
On the FN Model D page in the Vault, you can find an English manual for the gun as well as a parts catalog in French and Dutch with an excellent series of exploded diagrams showing every component part of the design.