I have been pillar bedding Mausers and Mosin Nagants with 3/8" tubing made from stainless steel, brass, and 1010 steel.
The Stainless is too hard to machine.
The Brass is a dissimilar metal touching the receiver.
I like the 1010 tube:
MSC Item Detail
I have been roughing the sides of the pillar for traction in the epoxy per Wagner's site: