You said "ream" but I'm pretty sure you meant to type "slug" the barrel. Reaming is a form of metal removal while "slugging the bore" is hammering and pushing through a soft lead slug so you can measure the lands and groove diameters.
The difference between .311 and .312 isn't enough to worry about as long as the jacket copper is soft and there's a nice soft lead center in the bullet. And in fact cast is generally selected to be a couple to three thou over the groove diameter. Hence why cast bullets for the .303 are .313".
If you want to slug the bore get a soft lead round ball from one of the local black powder shooters. A .45cal tapped to swage into the bore with a brass hammer will soon fit with a "washer" of lead on the crown. When you're that far in use a brass punch or a section fo cheap cleaning rod to tap the slug in bit further in so it breaks away from the crown "washer". From there you can simply push it through with a cleaning rod.
A micrometer is nice for measuring the groove diameter but I've always had to use a set of calipers to measure the land diameter. That's because the slug is the reverse image so the "grooves" in the slug are the lands while the lands on the slug are from the bore grooves.
The difference between .311 and .312 isn't enough to worry about as long as the jacket copper is soft and there's a nice soft lead center in the bullet. And in fact cast is generally selected to be a couple to three thou over the groove diameter. Hence why cast bullets for the .303 are .313".
If you want to slug the bore get a soft lead round ball from one of the local black powder shooters. A .45cal tapped to swage into the bore with a brass hammer will soon fit with a "washer" of lead on the crown. When you're that far in use a brass punch or a section fo cheap cleaning rod to tap the slug in bit further in so it breaks away from the crown "washer". From there you can simply push it through with a cleaning rod.
A micrometer is nice for measuring the groove diameter but I've always had to use a set of calipers to measure the land diameter. That's because the slug is the reverse image so the "grooves" in the slug are the lands while the lands on the slug are from the bore grooves.
