With the Lee-Enfield, the Proof testing required 2 rounds, 1 of which was a Q load (very high pressure) the other a Ball round which had been dipped in oil (to mate the locking lugs and recesses).
The rifle generally then was fired a few rounds for sighting. Once it was sighted-in, it was fired 5 rounds for accuracy. If it warranted it, it got 5 or 10 more. The best were put to one side and fired again to confirm what was suspected, then most went to Holland and Holland for conversion to T (Telescope) spec. THAT work then had to be tested as well.
So you buy a "brand-new, unfired, completely original Number 4 Mark 1(T)", in the box with all the goodies.... and it is in WONderful condition.
But it has around 30 rounds through it, very possibly more.
I don't think ANY made it out the door without at least 7 rounds through them.
When looking at a rifle, check the wear at the MUZZLE first, then check wear at the LEADE. You are looking for sharp, well-defined Lands standing up over the Grooves. Grooves should be as free from toolmarks as possible. There should be little to zero wear on the corners of the Lands immediately in front of the Chamber.
If you have that, then you should have a shooter.
Remember also, TIME is not on our side. Wood which was kiln-dried...... or even naturally-dried.... which was perfect when it was made into a stock, has had 70 years or more to lose fluid, to pick up fluid, to warp, to twist, in ways which you might not think even likely.
If you start with a good BARREL, you will have something which can be MADE to shoot.
With military rifles, I have found that generally you will get the best accuracy by ascertaining the military loading, then dropping it by 5 to 10% in velocity. This idea is not original to me; it was learned during the development of the Mark VII Ball .303 cartridge, 'way back in 1909.
Good luck!