Caliber and ram rod size have no relationship. 3/8" will work for everything down to .40 and every thing up to 10 bore. anything smaller than .40 will need 5/16".
The old guns never have a rod bigger than 3/8" except for the 1st model bess, which I believe may have used something near a 7/16", of course it's clunky archetecture doesn't suffer from a bigger rod.
I prefer a rod as large as the bore and thimbles will allow. A 7/16" rod is not only MUCH sturdier than a 3/8" but also much easier to grasp when seating a tight load down a dirty barrel or when pulling a ball. Of course the style of rifle has to be considered as well, 7/16" would spoil the lines of a slim longrifle but is right at home on a heavy half-stock. Even in my .36 I am using an 11/32" brass rod as it is more rigid and less slippery than the usually recommended 5/16".
