F-Class Calibers
Tate,
To some degree it depends on what class you intend to shoot in. There are two, F-Open and F/TR in the US NRA rules.
http://www.nrahq.org/compete/RuleBooks/HPR/hpr-w22.pdf
Section 22, rule 3.4 (b)
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
If you are planning on building a new rifle I would advise you to attend a good size F-Class match before you spend your money. Even if you are going to shoot in the Open class I think you are making a big mistake. You are most likely going to shoot 50-80 shots from a prone position over the course of a days shooting and the recoil even from a 22 lb rifle will lead to problems. Decreased accuracy from fouling may also become a problem as the day progresses. I have seen a few people shot well on occasion using 300 WSM’s but even that is too big for most. If you decide to shoot a 30-378 Wby buy extra barrels, you are going to need them.
P.S. The U.S. F-Class Open Team shoots 6.5-284 for the reasons stated.