Kcordell:
I would seriously doubt your barrel is a .318; by 1924 there were few new rifles being made in that caliber--even the famous Brit .318 Nitro, which, of course, shot a .330 projectile. My own 8x56MS 1908 Mannlicher is a true .323; the old Western 200gr RNSP's, which are the only commercial loads I ever had for it, miked at about .3215. It's not hard to tell if you have a micrometer (and if you don't, you'll soon need one anyway); and a plastic or wooden mallet (don't want to damage the crown of the barrel, you see). First, remove the bolt and the scope. Next, simply find a .36 to .40 round ball (one of your muzzleloading friends should have one, or you can take a larger one and beat it slightly oblong, or use a pear-shaped sinker from the local tackle shop)-- and drive it down the muzzle. A small piece of wooden dowel about 1/4 to 3/8" in diameter will help you get it down flush with the crown. Now drive it a bit further, if you can; sometimes bores are a bit looser just at the muzzle from cleaning-rod wear; I like to go about an inch deep. Then, with a cleaning rod, from the receiver end, tap the slug out onto a towel or other soft spot, measure it carefully at its widest diameter with the micrometer (you'll be measuring from groove to groove in the bore by measuring land to land on the slug) and voila! You've slugged and miked your barrel.
Oh, yes; now you'll want to clean the bore with a decent bore solvent. If this seems rather rough treatment, just think how you'd shudder if someone else, who didn't love it, did the same thing with a bigger hammer. If you've done this right, and the rifling has opposing grooves, you'll be as certain as the gunsmith, and not inclined to fudge the numbers for liability's sake. I'm guessing you'll read from .323 to .3245; don't worry even if it's .3255 or so, as jacketed slugs can be spun quite consistently from a slighty oversized bore.
Good Hunting! And mind yer topknot!
windy