Double Rifle Dynamics

i helped paul roberts one time when he owned rigbys by shooting a double .470 for the person he used to regulate his doubles. he looked like a wizard, and when i watched him working on the double rifle the more convinced i became. i shot 2 rounds through each barrel and we went to check the target. they were about 12 inches apart and the left barrel 6 inches low and the right barrel the same high. as i sighted in some scoped rifles he went to work using if i remember a small portable work bench, wire , small pieces of wood for putting under the wire , pliers , metal rods and a blow torch. he wrapped wire round the barrels and used the other things to alter the barrels very gently as he heated the solder, after 20 mins or so he gave me the rifle to shoot again. the point of impact had moved to parallel ,but maybe 6 inches apart. the same procedure followed a couple of times with him hunched over his small work bench as he manipulated the barrels using skill and judgement learnt over years. after putting i think 8 rounds( 2 at a time through each barrel), he had used what to me was magic to make that double place its shots about an inch apart at 60 yards. i heard he had died a few years later, and i thought of the knowledge and skill learnt over decades that went with him. i know that some doubles are fairly easy to regulate, and others for no forseable reason are a pain in the ass, and can cost a lot of money simply in ammunition used to try and sort them out.