I don't know what specifically people are using in 'custom' barrelled bolt guns in .223... Most people I know shooting 'heavy' bullets in a .223 bolt gun are using a factory barrel like a Savage (1-9"), CZ (1-9") or a Rem 700 LTR (1-9"), and had to pretty much take what they got as far as chamber. For AR's I think there are basically 3 'normal' lengths of chambers, for something like 2.450", 2.500, and 2.550" OAL, since they have to function w/ mag length ammo as well as long loaded slowfire rounds. the short stuff is still jumping a bunch, though, and the long stuff is pretty long. It might be worth asking over on www.longrange.com
(Long Range High Power competition, several folks from your neck of the woods there also) or Floridahighpower.com. I'm pretty sure there are some people who shoot or have shot bolt gun .223's for long range.
Sorry I don't have a direct answer for you.