Kiwi,
Sharps 4590 has given good advice, but you should also consider the following: If you install one chambered for a high pressure cartridge, you may have to "bush" the firing pin or have a Mann-Niedner?type pin installed(even if the action is strong enough.You would be limited to break open guns.You would have to provide a means to mount a scope or install iron sights.There are several firms that make full length insert barrels(Einstecklaufs, ELs)and they don't all use the same method of attaching the end,your gunsmith will have to have the EL "in hand" to determine what is required.You may find a shorter EL more convienent, but would normally be limited to a rimfire cartridge(22LR,22RFM,or now 17RF).I believe they are avaliable in 22mm and 44mm length(about 10"and 20")which can be pretty useful.
Mike
