300 Blk dies at .311

[vondelta]

Can anyone tell me why many people are using dies for the 300 Blk that cast boolits at .311 instead of the normal .309? The 300 Blk is a 30 cal firearm, which is .308 correct? So why .311 casts?

[GRUMPA]

.311 so they can be sized to what you want them to be. It's the final size that matters, this just gives the caster room to size to what they like best. I have a .308 bore that likes .310 stuff through it.

[DrB]

General experience is that around a thousandths over bore diameter shoots well/leads least, presumably because it seals the bore and prevents gas cutting of the projectile.

Most experienced folks here will tell you one of the first steps to getting your individual gun to shoot well with cast is to slug the bore and measure groove diameter, then size bullets around .001 over that diameter.

I have had good experiences shooting as cast two or three thousandths over groove diameter with some rifles (as long as they still chamber okay). The throat ahead of the chamber sizes the boolit down... As long as this happens such that the bullet ends up symmetric it seems to shoot well. If chamber concentricity, throat, lede, and bullet geometry are such that this tends to imbalance the bullet then shooting oversized can decrease accuracy as the bullet becomes more over bore diameter. Unintended/inconsistent neck tension from oversized bullets and mixed brass (the bullet may also be "sized" by the case neck) can also have a negative effect if you go too large. Again, most folks say around .001 over.

What works best in a particular as-built gun is dependent on a bunch of variables, but measuring your bore seems to be a good first step.

[stubshaft]

Depends on the bore size of your gun. If you have a dedicated mold then you can size .001" over bore diameter. If you have many guns that shoot the same caliber boolit then you could use an opversize mold to make boolits for all of them.

[runfiverun]

i just size my 30 cals to 310 if they show an issue then i dig for the problem.
310 is also pretty easy to seat in a normal die without issues.
i look at boolit shape and nose/body bearing length compared to throat shape long before i worry about diameter.
many of my cast boolits are shot as a slip fit in the barell,a good smooth barell, and are only .0005 oversized.
like 2245 in my 223,22-250,220 swift.
323 in my rem 8 mauser.
3575 in my dan wesson revolver,and 358 sized in my rossi 357 rifle.
some guns need a larger oversized boolit to fill the throats [like all three of my argies] a 314299 seated less than a cal deep won't touch steel in thier throats but can't be driven down thier bbl without some work.
i need a 215 gr tapered loverign design sized about 317-8 at the base and about 306 on the nose to fill thier throats, yeah the 314 shoots pretty good with the right load and at lower velocities, but the throat shaped boolit would do so much better.