I just bought a piece of "W" (letter size) water hardening (W-1) too steel. Milled a groove in it hardened, tempered and drilled a hole. Too short to wander.
Temper is not as blue as it looks the quench actually made the blue spots in the groove cutting edge was straw before drilling.
"W" is about .386" diameter. 3/8 rod fits just right.
I set it up to drill slighty up toward the breech of the barrel and away from the lock side.
I also made a new punch for making 3/8" rod pipes for part of it. Will need to extend it for long rifles it was only 36" anyway.
Had not used W-1 before but am very happy with it for this use. No oil to clean off after quench for one thing.
Barrel is 54 Douglas 1" at the breech and 7/8" at the muzzle. 9" long.
Dan