A lot of us serious MILSURP wankers do invest in the tooling necessary to overhaul/rebuild rifles incl doing barrel changes. This means a selection of gauges, barrel vices, receiver wrenches, stripping tools, taps & dies, and misc fixtures for the types of rifles we like to work on. You will never recover your investment on these unless you are doing a number of jobs. It just doesn't pay to pay to get the tooling to do a one-time project, so one must find a qualified 'smith to do the job and pay for his time and shop overheads. In many cases this may just not be cost-effective versus the comparative price of a rifle or a barreled action.
Rebarrelling a No1MkIII is a bit more challenging than doing the same job on a No4 Lee-Enfield. In the case of the No1, the critical consideration when installing and indexing a barrel is to have the breech end butt up tightly against the collar inside the receiver ring to lock the barrel in place. To a lesser extent the barrel shoulder should also butt up against the face of the receiver, but there are perfectly servicable No1s out there where there is a detectable gap between the barrel shoulder and the face of the receiver, the more critical fitting being the relationship of the breech end of the barrel and the collar inside the receiver ring. On a No4 the requirement is to get a crush fit between the barrel shoulder and the front face of the receiver with a starting point where the barrel is approx 17-18 degrees off vertical index when hand tight in on the receiver. Military shops normally had a good selection of barrels to choose from when rebarrelling a rifle, so the correct initial relationship of barrel to receiver was most often found by selective fitting of barrels, which do show differences because of manufacturing tolerances. If you are stuck with installing a barrel where the correct hand tight initial index cannot be found, then some lathe work will be necessary to adjust the initial fit of the barrel. You can also use a shim between the barrel shoulder and face of the receiver of up to 3 or 4 thou thickness if the barrel over-indexes when hand tight. I think that there are more folks out there who are experienced with fitting No4 barrels than No1 barrels simply because of the greater availability of No4 actions and replacement barrels, plus the preference for target shooters to use the No4.
Getting the right barrel/receiver relationship before turning the barrel into vertical index is important. A barrel which is too loose may well turn and become loose while firing the rifle-not a good situation. A barrel which is too far off index when at the initial hand tight position is also a problem as the higher torque forces necessary to index it can crack or distort the receiver or gall the face of the receiver.
