Sizing .30 luger lead bullets
 

I just slugged my .30 luger barrel. Groove diameter is .312. Is this the diameter I should size my cast bullets? Thanks, John

Hi John,
Welcome to the ward.

.312 would be about right for cast bullets. I shoot jacketed .312 Hornady XTPs in my .30s, they work very well and are very accurate.

Ron

Thanks Ron, I wasn't sure about the cast bullet size. Ordered a .312 die from midway.

Just curious,
Were the .30 barrels also stamped on the bottom with the bore size?
What was this one stamped, and how does that compare the the slug results?

Still Lernin',
Thanks.
Fritz.

bullet size
 

If the barrel is marked with a diameter it would be the bore diameter not the groove diameter.

If the barrel slugs at 0.312 then 0.312 or even 0.313 would be the ideal bullet size for the barrel. However there is a booby trap here. You must consider the outside neck diameter and how it relates to the neck diameter of the chamber. You must allow room for the neck to expand and release the bullet or your pressure may rise to uncomfortable levels. The original design called for a jacketed bullet smaller than the groove diameter but the chamber neck was sized for the cartridge with the smaller bullet.

So get a measurement of the neck diameter in the chamber and limit the bullet size so that the cartridge neck diameter will be at least 0.001 inch smaller than the chamber neck diameter. You may find the neck diameter to be the real limiting factor on bullet diameter.

Sieger tells me he has not run into a serious problem with this. But I will still say know what your neck diameter is.

BTW: I do have this problem converting 9.3x74R brass to 400-360 brass. I have to ream the necks.