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How a Fedorov Avtomat Works
The Fedorov Avtomat is a short-recoil operated, locked-breech weapon which fires from a closed bolt. The bolt locking is achieved by two dumbbell-shaped locking plates, mounted at either side of the breech, latching barrel and bolt together through lugs on the bolt. Those plates are allowed to tilt slightly down after about 10 mm of free recoil, unlocking the bolt. A bolt hold-open device is fitted and the firing mechanism is of hammer type.
Captain V. Fedorov began a prototype of a semi-automatic rifle in 1906, working with future small arms designer Vasily Degtyaryov as his assistant. A model was submitted to the Rifle Commission of the Russian army in 1911, which eventually ordered 150 more rifles for testing. In 1913, Fedorov submitted a prototype automatic rifle with a stripper clip-fed fixed magazine, chambered for his own experimental rimless 6.5 mm cartridge, called the 6.5mm Fedorov. This new rimless ammunition was more compact than the rimmed Russian 7.62×54mmR, better suited for automatic weapons and produced less recoil, however, the round was prone to occasional jamming. When fired from an 800 mm barrel, this experimental cartridge propelled a pointed jacketed bullet weighing 8.5 grams at an initial velocity of 860 m/s with a muzzle energy of 3,140 J as opposed to the 3,550 J muzzle energy of 7.62×54mmR ammunition from a barrel of the same length. 6.5 mm Fedorov rifles were tested late in 1913 with somewhat favorable results.
In the autumn of 1915, Fedorov was posted as a military observer to France, in the Mont-Saint-Éloi sector. Here he was impressed by the ubiquity of the French Chauchat and by the firepower it brought, but less so about its mobility. According to Fedorov's memoirs, it is here he came up with the idea of introducing into Russian service a weapon with firepower intermediate between the rifle and the light machine gun, but with mobility comparable to a rifle. His decision to adapt his semi-automatic rifle design for this purpose was one of wartime expediency. Fedorov set to the task upon his return to Russia in January 1916. He retained the mechanism of his semi-automatic rifle, with the major addition of a selective fire switch. The fixed magazine was replaced by a curved 25-round detachable box magazine. Due to limited trial nature of its production, most of its parts were custom fitted and not interchangeable, including the magazine. Therefore, in practice, the Fedorov was issued to the troops with only one magazine, which would be reloaded through the breech via standard 5-round Arisaka stripper clips.
Production of the new cartridge was out of question so it was decided to convert 6.5 mm Fedorov rifles to use the Japanese 6.5×50mmSR Arisaka ammunition which was in abundance, having been purchased from Japan and Great Britain along with Arisaka rifles. (About 763,000 Arisaka-type rifles were imported to Russia, along with approximately 400 million cartridges for them; domestic production of the Arisaka cartridge remained insignificant though.) The change of ammunition involved only minimal changes to the rifle, including a chamber insert and a new range scale for the rear sights. The somewhat less powerful Japanese cartridge meant that the muzzle velocity was only about 654–660 m/s because of constrained barrel length.
A US Army analysis from the early 1950s considered that the Fedorov Avtomat was unreasonably complex to manufacture and that it suffered from rapid overheating of the barrel on automatic fire. Russian tests indicated that the gun could fire about 300 rounds continuously before heat buildup rendered it inoperable. This was still an improvement compared to the Mosin–Nagant M1891 rifles, which would begin to smolder after 100 rounds. The main factor in the increased heat dissipation was the metal shroud over the barrel at the end of the forearm, which acted as a radiator. In terms of accuracy, Russian data indicates that when fired in short bursts the Fedorov Avtomat could reliably hit targets having a profile of 0.6×0.5 m at a distance of 200 m. At 400 m the dispersion increased to 1.1×0.9 m and at 800 m it was 2.1×1.85 m. Consequently, burst fire was only considered effective up to about 500 m.