This is a perennial issue.

Original No4s of all flavours were tuned to Mk7 ball.

7.62 NATO produces a completely different vibration and "whip" regime, even with an identically profiled barrel. All decent military rifles are tuned to the ammo, not the other way around.

So, if you manage to get your barrel to cut 2" groups at 100yds. with NATO spec ball, do NOT expect it to shoot one hole groups with MatchKings or VLDs. Most 7.62NATO spec barrels have a RH twist of 1:12". Long, super-slinky bullets will either NOT stabilize at all, or will precess wildly for the first few hundred yards whilst trying to settle down.

And all this assumes that the fore-end is seated perfectly on the draws and all the other normal Lee Enfield caveats. (See Mr Laidlers detailed instructions posted elsewhere on this site).

And don't forget headspace and locking lug bearing.

The L42 series perform astoundingly well with the ammo for which they were intended: Same action, different barrel and fore-end (complete with very nifty extra Plate, Seating (1005-99-963-1592) to spread the load from the front screw.