I don't "cling" to the 308, I use the chambering required per the NRA rule book for F-TR
3.4 F Class Rifle
(b) F-Class Target Rifle (F-TR)- A rifle limited to .223 Remington or .308 Winchester or their commercial and/or metrification equivalents. Chamber dimensions must not exceed the maximum SAAMI or CIP specifications. Bore, groove, and throat dimensions may be selected to suit bullets used. There is no restriction on bullet weight. The rifle must be fired off a bipod, rigidly ttached to the rifle’s fore-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.
What you refer to is F-Open. I choose not to shoot in Open for a number of reasons.
http://competitions.nra.org/documents/pdf/compete/RuleBooks/HPR/hpr-book.pdf
The F class rules start on page 67