Sizing .30 caliber cast bullets

[DickelDawg]

What difference does it make whether I size .30 caliber cast bullets to .308 or .309 for my 30-06 rifle?

[HATCH]

It depends on your barrel.
Most people size .001 larger then the caliber. So .309 for 30 cal.
I size all my cast boolits this way. .358 for my 38s and 357s
.356 for my 9mm.... Ect
You want a good fit. Lead will compress to fit the bore some so being oversize by .001 will not hurt anything

[DickelDawg]

Thanks, Hatch.

[Yodogsandman]

Your boolit needs to be supported during initial release from the case and entering the bore during firing. It gets kicked in the butt at lets say 45,000 PSI when fired and tends to bend and squash (obturate). Sizing to the throat will ensure that it's supported. The throat is the area between the case mouth and leade going into the bore. The boolit will conform to your bore if it's a little oversize as HATCH said. Undersized boolits might also lead your bore, too.

For best accuracy, slug your chamber and size to the throat dimension -.0005" for best accuracy. See....

http://castboolits.gunloads.com/showthread.php?218414-FYI-This-is-how-I-do-a-pound-cast

If you don't want to do that, check the SAAMI throat dimensions for 30-06. That should get you close. (.3105"). So, minus the .0005" would be .310".

[ShooterAZ]

It will vary slightly from one gun to another. Sometimes you have to experiment a little for best results. My 30 carbine wants .309, my 300 Blackout AR likes .310, my Remington 700 30-06 and my 30-30's like .311. 7.62x54R Mosin likes .313, and so on.

[TXGunNut]

My default size for 30 caliber is .311. If a dummy round chambers I'm good to go. If it doesn't, I'll try .310. Char-Gar wrote an excellent article awhile back explaining why chamber dimensions are more important than bore size for boolit sizing and nose design.

[Screwbolts]

I agree with post 6 , I size to the larges diameter that will chamber freely. The barrel will finish sizing it to what is best for it.

[Outpost75]

To determine the correct bullet diameter for a rifle, the groove diameter of the barrel is NOT the determinant.

INSTEAD you want to measure the throat, or the unrifled portion of the barrel forcing cone or "ball seat" ahead of the case mouth, before the rifling starts. The best way to do this is from a chamber cast or upset throat slug.

If you forget EVERYTHING you ever read about slugging barrels and simply cast chambers from now on, and get bullets to FIT THE THROAT you will be far happier in the long run.

The limiting factor in bullet diameter is neck clearance. So you must measure the neck diameter of the chamber on the cast, then the loaded cartridge neck diameter must not be larger than 0.0015" SMALLER than the chamber cast at that point, to ensure safe expansion for bullet release.

In a typical .30-'06 SAAMI chamber min. chamber neck is 0.340". Typical case mouth wall thickness of commercial '06 brass is 0.013," so .340" minus twice neck thickness (0.026") = .314", minus 0.0015 for safe expansion = .3125" max. bullet! IF your throat casts that large. (Hint - if the gun has been shot alot, it probably does!).

In the SAAMI Minimum chamber the diameter of the forcing cone entrance is .310" and proper bullet diameter for the MINIMUM chamber is .3095". In most new .30-'06 sporters, if you presume the mean assembly tolerance, a .310" diameter fits best. In worn chambers which have been shot a bit, or military chambers as in the 03A3 Springfields, a diameter of .311" is better.

Very worn barrels which have been shot a great deal will shoot best with a .312" bullet IF it chambers and extracts freely in your rifle.

[Char-Gar]

What difference does it make whether I size .30 caliber cast bullets to .308 or .309 for my 30-06 rifle?

Probably very little if any difference,for most likely, both those diameters will be too small for optimal accuracy.

[DickelDawg]

Thanks to all of you for the detailed and very good information. I appreciate it.