Roger,

  I've never done it but have had several liners made up by turning barrl blanks down, and my gunsmith regulary turns down barrels such as you want with no problems at all.

  Most of the time there shouldn't be any problems especially if the barrel was stress relieved. Which it should be!

  But, as Green Mtn. suggested to you there is a possibility of the barrel warping on you. One that comes to mind is what one looked like after my gunsmith got done turning it down. You could actually see the warp it was that far off. And not just in one place. Looked almost like a corkscrew.

  So, it comes down to you takes your chances, and hope for the best.

PETE

Green Mountain bbls are stress relieved twice, so there should be minimal stresses left in the bbl. Heat is the thing that will cause problems in turning down, so take all possible precautions to avoid heat buildup. This includes but is not limited to light cuts & slow spindle speeds, and coolant would be a big help as mentioned above. Definitely use a live tallstock center to avoid more heat buildup.

Another precaution I commonly rely upon is to leave the lathe tailstock clamp very slightly loose. Usually we turn bbls between centers because of the required taper, and so we must allow the headstock end to rotate in line with the offset tailstock. The turning action will build up a little heat, which will cause the bbl to grow slightly in length. If it's trapped between rigidly fixed centers then there's no place for it to go except sideways, & voila! then we magically have a warped bbl.

So I take care to leave the tailstock clamp snug but not cinched down rigidly. The amount of clamping force should be such that the act of turning won't cause the tailstock to retreat under the turning pressure, but will allow it to retreat slightly when the pressure increases due to the bbl's length increasing. This is a delicate balance but an experienced operator should have no trouble. A little experimentation will soon show the proper amount of clamping force.

In the past I've turned GM bbls to 1/2 round, and had no trouble at all except for tool chatter. A steady rest will solve most of the chatter, as will using a specially-shaped lathe cutting tool. The details of how the anti-chatter tool is shaped can be found in any of several machinist's textbooks but most experienced operators already know about it.
I suggest you choose your operator very carefully, good luck, Joe

I have a .40 cal GM barrel 28" long half octagon / half round and do not notice any ill effects in either looks or performance. It was turned by gunsmith Steve Durren in Michigan.

Glenn,

The middle half.

Steve Durren

FWIW,  Steve Durren has turned down two GM barrels for me,  my 38-55 has a more or less half-round half octogon and it shoots a lot better than I do I'm sure.  he just turned dow another in 40-70 SS for the english style rifle I'm creating with my wesson/Earle #1