The small pin that passes through the fore end beneath the barrel holds it in the stock, along with the tang bolt. You also must remove the ramrod and the crossbolt that holds the lock in place as they otherwise can interfere with removal of the barrel. Even though the rifle may be a "Custom-made" gun, if the barrel is from a commercial maker then it will definitely be stamped with the name and rifling twist rate and likely that will be found on the bottom barrel flat at the breech. That little retaining pin should slide out quite easily if nudged with a slightly undersize pin punch.