Doing a "drop-in" barrel will present a few challenges, especially if the receiver threads have been a bit "stressed". I understand that in the later days of "service" No4s, there appears to have been provision for two grades of barrel: "fit to new" and "fit to used". An advance offset of about 11 degrees of the breech thread on the tenon was the only difference. Torque in the barrel and then choose a bolthead to headspace.

It's a bit of a tap-dancing act with the No4 barrel because of the front sight and bayonet lugs. If you cut the breeching shoulder a bit short so that it can be adjusted to achieve correct torque and alignment, what effect will that have on the headspace?

The civilian market is nothing like an ordnance system in which the armourers, with their gauges, have final say on whether to rebarrel or not. If a receiver is "past it" then, as long as the receiver is destroyed by the authorised folk and paperwork is done correctly, that's it. Not many civilian shooters would be too happy with that.

A better solution is an "almost finished" barrel that requires the attention of a good gunsmith to index and headspace. At least with No4s you have multiple bolt head sizes to play with. Doing replacement barrels for SMLEs is a bit different. Nothing that a competent, Lee Enfield savvy gun plumber can't handle.

Then again, if your bolt is dancing around in the receiver like a frog in a sock, it may be time to get a new rifle.

A while ago I got some .303 blanks made by my local barrel maker. They have 1:10: LEFT HAND, 5 groove rifling. The rifling form is a variation on 5R form found in match barrels. Bore is nom. .3025", groove is .311"; a bit tighter than the original spec. The first couple went into P-14s and shoot quite well with Yugoslav ball and various handloads. A couple of "bitza" SMLEs are being lined up for the treatment soon. Still fine-tuning the process for the No4 barrels; Those little lugs at the front tend to complicate things.