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F class comes in 2 flavors, all shot prone, and at at least 600 yards. on an MR1FC target
F-open (F/O) which allows anything .35 cal and under, any scopes, and sleds or supports of most any kind. Fast flat-shooting calibers and highly customized "race" guns abound here. (6mmBR, etc.).
F Target Rifle (F/TR) can only use military calibers, essentially .223 or .308 (or NATO equivalent), and usually involve more spartan tactical rifles and accessories (fixed bipods), and often lower powered scopes (10X Mildot for instance).
A military/service rifle class, with open sights and slings are also fired at any/any shoots, and use the larger MR1 target.
Full rules:
F-Class Target Rifle (F-TR) - A rifle restricted to the chambers of unmodified .308 Winchester/7.62mm NATO or unmodified .223 Remington/5.56mm X 45 NATO cartridge cases. The rifle must be fired off a bipod, rigidly attached to the rifle’s for-end, and/or a sling. Any bipod, meeting the definition of a bipod, may be used but its weight must be included in the rifle’s overall weight. Any safe, manually operated trigger is permitted. Any sighting system is permitted, but it must be included in the rifle’s overall weight.
The provisions of Rules 3.16 and 3.16.1 apply to this definition.
(1) The rifle’s overall weight, including all attachments such as sights, sling and bipod, must not exceed 8.25 kilograms (approximately 18.18 pounds. An “attachment” also includes any external object, other than the competitor and apparel, which recoils or partially recoils with the rifle, or which is clamped, held, or joined in any way to the rifle for each shot, or which even slightly raises with the lifting of the rifle from its rest/firing point.
(2) The rifle must be fired in the prone position from the shoulder of the competitor using rifle as defined in 3.4.1(b).
(a) F-Class Open Rifle (F-0) - A rifle restricted to a bore diameter no larger than .35 caliber. (Attention is directed to safety fan limitations of various ranges. Individual ranges may further restrict ammunition), “Rail guns” and positive mechanical methods of returning to the precise point of aim for the prior shot are not permitted. Any safe, manually operated trigger is permitted. Any sighting system is permitted, but it must be included in the rifle’s overall weight.
The provisions of Rules 3.16 and 3.16.1 apply to this definition.
(1) The rifle’s overall weight, including all attachments such as sights and bipod, must not exceed 10 kilograms (approximately 22 pounds). An “attachment” also includes any external object, other that the competitor and apparel, which recoils or partially recoils with the rifle, or which is clamped, held, or joined in any way to the rifle for each shot, or which even slightly raises with the firing of the rifle from the rests).
(2) The width of the rifle’s forend shall not exceed 76mm (approximately 3 inches).
(3) The rifle must be fired in the prone position from the shoulder of the competitor using rifle rests as defined in Rule 3.4.1(a).
Under American F-Class Open rules