Lynn,
I dont currently have a Ballard with Rigby flats but have done a number of them.  When you get the dimensions, you can whittle most of the material away with an end  mill.  You will be left with a file cleanup.
Barrel will already have been threaded to fit up the action.  Make a bushing internally threaded to fit the barrel shank threads.  Externally, the bushing wants to continue the taper of the round section of the barrel.  Harden the bushing and use it as a filing guide to keep the back of the barrel round and edges crisp.  You will be able to file/polish right up to the edges of the flats.
Jim

From a JM Marlin 6 1/2 Rigby Offhand Rifle

Length of top flat:   2.20"

Width of top flat: .367"

Width of side flat:  .221"

Width of barrel at end of flats:   1.031"

Hope this helps.

Regards,
Powderman

Width across flats at receiver/barrel junction:  1.145"

Width across flats at round end of flats:   1.130"

Width at end of 28" uncut .38 calibre barrel: .886"

I used to have 3 Rigby Ballads with 30" barrels, but sadly no longer. Perhaps someone with a 30" will chime in.

Regards,
Powderman

I want to again thank powderman and then make an offer to anyone interested. 
Using the dimensions provided I have created a 3D solid model of a Ballard Rigby barrel. I made this one bored at .400 and then rifled to a groove diameter of .408 at an 18" twist. I then chambered it for basically a 40-63. The Rigby flats are to the dimensions provided withing .001" (I rounded as needed to keep the math simple  ) I crowned it with a recessed diameter and then a 45 degree chamfer. The shank is to the dimension of a Ballard and the threads are 18 TPI and not meant to be exact for thread depth dimensions, just representative.
Anyway the purpose of this was to create a model of the flats, so if anyone is interested in a copy of this 3D solid model in a .STP file format please PM me.