Originally Posted by
Char-Gar
Let's do a short primer on bullet fit, because I don't think you have it yet.
1. The bore size is almost irrelevant with cast bullets. You can push oversized bullet down the barrel with no negative consequences.
2. The critical dimension is the barrel throat. A bullet needs to fit the throat as close as possible without shaving lead when it enters the throat. A clearance of .0005 to .0001 is all you need. The reason this is important is because a cast bullet that is a good fit to the throat will enter the barrel straight and not cockeyed.
3. If you use a bullet so large the case neck it to large to chamber the round freely then you have another problem. The loaded round need about .0005 to .0001 to allow the brass to spring away from the bullet, freeing it to go on it way into the throat and barrel. If this clearance is not there, high pressure can be induced as the brass can't spring away from the bullet.
To recap...
1. use a bullet that fills the throat but does not shave lead.
2. As long as the loaded round doesn't chamber hard due to an enlarge neck, you have no problems.
3. In a few cases, to get a bullet large enough to fill the throat and not enlarge the neck too much, you need to neck turn the brass a little to reduce it's thickness and have the proper clearance.
You don't need a huge number of molds. A standard 30 caliber mold should throw bullets .311 or smaller to be sized to fit the throat of most commercial 30 caliber rifles. .311 to .310 will be right most of the time. An occasional rifle with a worn throat will do best with a .312 bullet.
The Krag with it large throat needs a "fat 30" caliber mold that throws bullet .314/.313 to size to fit larger throats like the Krag, the Mosin, 303 Brit and 7.64 Belgian/Argentinian.
The old mantra is "slug the bore" needs to be discarded as the critical dimension is the barrel throat/freebore. The throat will provide the size limit and not the bore.
If a fellow is looking for cast bullet accuracy in his rifle, he needs to understand bullet fit. There are more points and issues, but the above is a good place to start.
BTW....Krag barrel groove diameter will run .308 to .314 with .3095 being the most common, so your measurements sound good to me, not that it matter, which it doesn't. My Krags run .308, .3095 and .312 and I shoot .313 bullets in the all. In fact I use the same ammo in them all.