I unashamedly cadged this excellent post from Jouster Enfield Forum just now- what more can anyone say.
Headspace on an Enfield is measured using British gauges: the minimum is supposed to be .064", which is the rim thickness of a military round, and .074", which therefore allowed .01" tolerance (in wartime, this tolerance was further increased to .084").
You will see that there is a lot of fuss by people who believe that their Enfield is out of spec. This is because the SAAMI gauges retailed by several manufacturers are different to the British gauges, and give a false reading. Part of the reason for this is that commercial US .303 brass often has a thinner rim than British or other manufacture.
Despite what you may read elsewhere, headspace does not cause a safety issue. Consider the bald facts that c.12 million Enfields of all models were made, and there is not - to my knowledge - even one case of one "blowing up" whilst using normal ammunition.
The "accuracy" aspect is debatable: target shooters, particularly in Australia, did indeed measure rim thickness and try and tighetn up the headspace - usually to .067", which is the optimum measurement. However, the rifle do not neccesarily react to tighter headspace, and can in fact become less accurate. I've tested dozens of rifles, swapping between long and short boltheads, and there is rarely any significant improvement. The overwhelming factors that determine accuracy are (1) correct stock fit (2) barrel condition (3) shooter ability. No(3) is significant: I've witnessed a sample of about 350 people shooting Enfields; only about 10% of those shooters were capable of the tight grouping from which other factors can be deduced. Ergo, for most people, tinkering with the rifle isn't going to help.
Other small warnings:
(a) the only reason an Enfield usually needs a change of bolthead is if it has been rebarrelled or ahd a new bolt fitted. The rifles were designed to be fired with wet ammunition and cope with extreme conditions. Most Enfields are still perfectly in spec with the bolthead they left the factory or service with - 100+ years ago, for some of them!
(b) over-tightening the headspace, by using the wrong gauge and then buying a longer bolthead, can cause premature wear to the rifle lugs.
(c) When an Enfield military armourer came to fit a new barrel, bolt or bolthead, he had boxes of spares from which to pick suitable components. This is vital, because the bolthead also has to be fitted for "over-rotation" as well as for length: if the bolthead rotates past the locking rib by more than a certain amount, then the recoil forces may be passing through the screw thread, rather than the bolt body. Civilian owners who go out and buy a single "longer" bolthead by mail order obviously take a chance that it will leave their rifle in spec - I suspect that most have in fact put their rifles "out of spec".
Before you go tinkering with your rifle, deduce what its best accuracy is, and if in fact it has a problem. Most No4s in average will shoot a 4" group at 100yds with any old milsurp or new commercial ammo. Many will shoot much better than this. Its always a good idea to obtain several types of .303 ammo, and see if your rifle "likes" one more than the others. Most Enfields will shoot best with flat-base bullets in milsurp MkVII ammo.
Most No4s can have their accuracy improved by just checking that the stocking-up is correct as per standard military issue. Sometimes a small corrective adjustment can dramatically improve grouping ability.
