Both the RCBS and Lee 7.62 x 39 dies come with two case neck expanders, one for .308" and .310" dia. bullets. The .310 expander ball or rod depending on the die set you have will work just fine for .311" dia. jacketed bullets. On the occasions that I or my brother do load a jacketed bullet larger than .308" the provided .310" expander has worked just fine.

There are some options if the standard expanders that come with several die sets, just don't cut it for you. One is using two-step case mouth expanders made by Lyman or NOE, those are two that I know make a two-step expander for 7.62 x 39. The size you choose depends on the dia. jacketed or cast bullet you are going to use. The Lyman version is called the M die, using either the M1 or M2 die body and the appropriate two-step expander plug which best matches your bullet's dia. on the list. Lyman pistol dies already come with a two-step expander, while you will need to purchase the die body and expander or expanders for rifle calibers.

NOE also makes both a solid and a powder through version of the two-step expander that is a drop in fit using the Lee Universal Flaring die, you just use the NOE expanders in place of the ones provided by Lee, to get the same effect as using the Lyman M die. I use both the solid and powder through versions for all my cast bullet rifle loads. There are two numbers associated with the expanders, the first number which is the largest expands the case mouth, the second number expand the case neck. In general, depending on the bullet's dia. say I'm loading a .314" cast bullet in 7.62 x 39 I want my expander to be .002" larger on the first step and .002" smaller for the case neck dia. so the expander I would choose would be .316" x .312" NOE has lots of options on these expanders so it's your choice as to what dia. you choose.