What would be considered the minimum wall thickness at the muzzle on a chrome moly barrel if I were to do a re-bore.

There is very low pressure at the muzzle so let your rebore guy tell you. Look at some double rifles; paper thin muzzles; I can get some measurements if you want but we have discussed this a lot, here.
I would just go for a new barrel; disclaimer; I contour and sell them. Mine have a muzzle OD of .660, so that gives a wall of .125; way more than enough for strength.
Balance and recoil will be the important factors.

JES Reboring recommends at least .200 over groove diameter. The 400 Whelen has a groove diameter of .411 so you'll want a minimum of .611. Mine is .675 at the muzzle.

My 450 NE Double has muzzle wall thickness of .080. That would make a single barrel very muzzle light, but not unsafe.

not to sure i want a rifle shooting heavy bullets to light weight

It has two barrels and most of the weight is in the middle; that is the way English Double rifles are designed. So they need thin muzzles to get the right balance. For a bolt action, obviously an .080 thick would be too light at the muzzle end.
The OP asked for "minimum wall thickness". That is not all that needs to be considered.

I was just trying to ascertain what minimum was for safety. I do like a little muzzle heavy balance in a bolt gun. Had the thought of re-boring an FN 30-06. I now know the muzzle diameter and would have to cut barrel back to 22" to have enough meat at the muzzle. I think this was just a pipe dream so to speak.

Just for safety, they can be just thick enough so the rifling doesn't show through. Chrome moly steel is very strong. Look at the experiments done by Whelen on Springfields; and those were not even chrome moly steel.

I’ve seen 9.3x74r double rifles with muzzle wall thickness down to .065”...just for reference.

Minimum thickness of a contoured blank vs minimum thickness for a rebore are going to be different.

Most that rebore have told me they will not rebore a barrel with walls less that .125" to .150" thick. If contemplating rebore, go straight to the source and ask what their minimum is.

I don't blame them as they don't know what kind of steel they are dealing with, whereas, a new maker has complete control over that.

quote:

Originally posted by dpcd:
I don't blame them as they don't know what kind of steel they are dealing with, whereas, a new maker has complete control over that.

I'm told it has more to do with the rigidity of the setup for the boring & rifling process than safety concerns. Makes sense to me.