7.62
Any of the .30 cal will fit very well.
7.62 base, and the part that holds the powder is rather large- about 1 cm in diam...
7.92 works good.
Recently, I have been cutting off the necks of .223 and/or 5.56(almost identical).
Its near perfect fit. Literally. And the longer brass adds wieght, allows me to add some lead, and also keeps the shaft from breaking right after the point.
If you cut off the base of the cartridge(primer, etc), and then slit the neck, it makes a cheap and easy ferrule.
44 mag work really good on Ceder arrows.
They add some weight...