9mm Range Officer - Bullet Diameter?

What bullet diameter is needed for the 9mm Range Officer?  I plan to order the 115 grain SWCBBs.  The standard size is .356" but they have optional sizes.

PS - Some of the writings on the Penn website are quite interesting, at least to me.  They even discuss loads for some of the bullets they manufacture.

I believe most of the Border Patrol guys shooting in the MPCPA here are using Springfield 1911's and using .357 dia bullets cast.

I attempted to slug my barrel this afternoon and came up with 0.355".  Does this sound about right?  The method used was ...

Melted a swagged .45 bullet in a 9mm case.  Then used a bullet separator to remove the slug.  Drove the slug through the barrel from the muzzle end with a wood dowel.  Measurement is tricky but the maximum found was .355".

So, knowing this which bullet should I use; .355, .356 (standard), .357 or .358?

Lead bullets are generally used .001-.002 over bore size, so a .357 lead bullet would be about right depending on the gun. Imho

Dardas will let u send a slugged bullet and measure it correctly for the gun and make recommendations.

Your RO is just the same as every other 9x19 pistol and will have the same variations in dimensions.
I would order 0.357" lead bullets myself. I find that my 9x19s all prefer somewhat larger bullets over nominal diameter (0.356-0.357" jacketed and 0.357-0.358" cast); however, each gun can be very individualistic in what it prefers.