I have a US Model 1917 rifle with the original 5-grove, left hand twist barrel that has been shortened to 22". I've slugged the barrel and as near as I can tell (by carefully rotating the slug inside the jaws of my caliper) it is .311" groove diameter. I've tried both .308" and .311 diameter bullets without much success. At the muzzle the lands of the rifling look fairly worn down, and I determined that I can stick the smooth shank end of an "N" drill bit (0.302") about an inch into the muzzle. So, not a good sign if the bore diameter was supposed to be 0.300" originally and is tighter 'up stream'. I'm wondering if there is a way to gauge the bore diameter over the entire barrel length? Does anyone make a set of plug gauges of varying sizes (say, incremented up in 5/10,000ths of an inch) that could be mounted on a cleaning rod and slid down the barrel to determine what will pass all the way through, what won't, and if there are any tight spots along the barrel? I've searched the web and cannot find any such thing.