Recutting base area has worked for me have also removed gas check to make a plain base bullet.
Never put a tool in deep enough to turn bore riding area maybe if I had dig. readout .
What I have done in this area is to cast a bullet ,drill and tap the base(leaving tap in the bullet) .cut vertical groove in the bullet and use lapping compound in the area to be enlarged and spin the bullet in the drill press .  Has worked for me.

Dales

Cat_Whisperer,

Like Dales, have opened up some bases to eliminate the gas checked area and get good fit, using a very short and large shank boring-bar (to minimize tool flex) with appropriate small tip size and shape.
Haven't changed the bore-riding area ahead of the forward grease-groove, as believe it would align as well as a tapered bullet does during breech-seating, but with fixed loads, may be beneficial to do so.

Cat,

I use some different length plunger-type dial indicators with magnetic bases that I use for depth control when doing critical boring/drilling. You can put them at various locations on the bed of the lathe, with the plunger preloaded against some spot of the carriage, so that when you move the carriage, you can closely read and control the travel.
Let us know how it works out for you.

One other thing........ you need to be careful dialing in the mold. If the mold halfs have any off set, when you push on that area, you'll push the mold halfs away from each other (see my thread on fixing out of round molds) making your cuts out of round of the mold. 

The other thing is the axis of the mold cavity has got to be inline with the lathe carriage, or the nose area won't be inline with the base. You need a pin with a push fit on the small dia of the mold, that protrudes out enough to indicate that axis for that length so, you know that the mold cavity is inline with the carriage.

Frank

cat_whisperer, I have done this many times. Learned early on to confine myself to modifying BASE BAND diameters only. Have a few that are only good for fishing weights where I tried to get inside and modify bands deeper in the cavity. When chucking in 4 jaw I shim the blocks on all 4 sides to protect them from Jaw damage when pushing the mold around with the jaws. Also use some fairly thick stanless shim stock .025 thick or better on all 4 sides. Also try to get indicator readings running true in two places at the same time.
This basicly guarantees true concentricity all the way in the mold. And a good square base. HTH Regards, FITZ.

I have lapped quite a few molds by spinning a drilled bullet in the mold.  I usually just do it to polish them to make them drop better but I have enlarged a mold up to .002.
I chuck up a bullet in the lathe and drill a hole in the base the right size for a sheet rock screw, one long enough to have an unthreaded section near the head.  I cut off the head with a wheel in the dremmel tool. Start the hole with a center drill.  I spin the bullet with a cordless drill.  If you want to lapp out the bore ride area only, file off some of the body diameter before you add the compound while spinning it.  My neighbor claims he has enlarged a mold nose up to about .005 this way.  180 grit works pretty fast.  Make sure you keep the mold faces clean so they will close completely, otherwise you might end up with an out of round bullet.

John Shaw

JLouis,

It is my firm belief that a person will never learn what can or cannot be done unless they are willing to try.
They may gain knowledge by making the attempt...  they won't if they don't.