Generally, when compared to the 6.5 Creedmoor, I believe that the .308 Winchester is disadvantaged by reason of its SAAMI dimensional specifications. The differences in SAAMI specifications between maximum and minimum permissible dimensions are greater for the .308 Winchester than for the 6.5 Creedmoor. For cartridge chamberings in wide general use, such as the .308, the larger commercial rifle manufacturers will keep their production tolerances within those SAAMI specifications, if they know what’s good for them. Otherwise, their customer service departments will be constantly dealing with warranty claims that a “defective” rifle excessively bulges cases or won’t chamber the bargain ammo the user bought on the cheap.
Chamber reamers for large scale commercial production of rifles chambered for the most common rifle cartridges, such as the .308 Winchester, are typically supplied at SAAMI maximum dimensions for a particular cartridge. Due to manufacturing cost considerations, those initial maximum dimensions are slowly reduced over time, through inevitable wear and resharpening, until they reach minimum SAAMI dimensions, after which the reamers are replaced. So, any particular rifle may have a chamber ranging anywhere from maximum (early in reamer lifespan) to minimum (late in reamer lifespan) SAAMI dimensions for its intended cartridge.
At the .308 maximum SAAMI chamber dimensions, a SAAMI minimum dimensioned cartridge would likely not be held consistently concentric to the bore axis upon firing, resulting in pretty ho-hum accuracy. Neck sizing of once fired cartridge cases can remedy that to a great extent. But, precision shooting with factory ammunition is most likely doomed. With such a mass produced rifle, then, the only hope is to luck into one which by chance happens to have chamber dimensions precisely matched to the dimensions of the factory ammunition to be used.
Even after all that, SAAMI chamber freebore specs for the two cartridges are quite different. The freebore dimensions won’t be as snug around a bullet in the .308 Winchester chamber as they will around a bullet in the 6.5 Creedmoor chamber. The 6.5 bullet is therefore given a much better chance to enter the rifling straight on, without yaw, than is the .308 bullet.
Custom or semi-custom rifle barrels with match chambers in .308 Winchester are quite a different story, however. And, there are those rifle manufacturers which produce the chamber and bore simultaneously through hammer forging, like H&K and maybe Ruger. Then, it comes down to the dimensional consistency of the ammunition, which can be a good deal looser in the vast array of commercially produced .308 Winchester cartridges.
