I've been wanting to write an article on headspace for some time now however I recently found an easy to understand article written by Parashooter on http://parallaxscurioandrelicfirearmsforums.yuku.com/
Instead of re-creating the wheel I'd like to share Parashooter's words with you here. Hopefully this answers a few questions and raises a few more.
I've included a few of my words at the end of Parashooter's article.
Several generations of American shooters have been convinced by bad information that something mysterious and scary called "headspace" should be checked and re-checked on almost any surplus rifle, especially Lee-Enfields. The truth is less interesting but still worth knowing.
Stripped to its essentials, with a rimmed cartridge like the .303, headspace is simply the distance between the front of the bolt and the back of the barrel. It's the space where the "head" (rim) of the cartridge fits when the rifle is loaded.
In addition, Lee-Enfields and their ammo were often made with a fair amount of space for dirt, mud, snow and other battlefield debris between the chamber and the cartridge's body and shoulder ("Body/shoulder clearance"). Since the cartridge is controlled by its rim, this clearance doesn't do any harm (except to handloaders who insist on full-length sizing).
When a full-power .303 cartridge is fired, a whole string of events occurs.
Click the animation below
2. The primer detonates. If it's not heavily crimped in place, it backs out, shoving the bolt and barrel as far apart as it can.
3. The thin, forward part of the case expands to fill and grip the chamber while the bullet moves out of the case and down the barrel.
4. The solid case head can't expand and grip the chamber, so it moves rearward, re-seating the primer, stretching the case walls just forward of the head, and stopping when it hits the bolt face. (In rear-locking actions like the Lee, the bolt and receiver also compress/stretch to add a little more movement. The higher the pressure, the more they move.)
5. If (and only if) the amount of head movement exceeds the elastic limits of the case, the cartridge separates into two pieces.
New cartridge cases can normally stretch a lot before breaking. Even with a minimum rim .054" thick and maximum "field" headspace of .074", the resultant .020" head clearance is well within the limits of new brass and it's very unlikely a new case will separate even if the headspace is somewhat more than the field maximum (which is pretty rare).
OK, but if one does separate I'm in deep trouble, right? Not really. It seems the short "cup" left behind the break is pretty good at keeping most of the gas where it belongs. Here's a demonstration -
First I took a case that had been reloaded with heavy loads enough times so it was stretched near breaking.
The point of all this is that excess headspace, even a bit beyond normal limits, isn't the terrible danger we've heard so much about. It's not a good thing for consistent ignition or long case life (although handloaders who neck-size or adjust F.L. dies carefully can control this) - but it's not a disaster waiting to happen.
Unless you're consistently getting broken cases when firing new ammo or brass, there's not much reason to be worried about headspace in these sturdy old Lee-Enfields. Relax and enjoy!
Editted to add:
It is unacceptable if you are experiencing case head separations with your lee enfield. Advise your section commander immediately (even if this has only happened once). The rifle needs to be checked out. If you are shooting WW2 / Korea vintage MkVII ammunition be aware that older brass can be a little bit more brittle. If you know that you have long headspace reconsider shooting this ammo - especially if the ammo looks tarnished or has signs of oxidation (stay with the newer Mk8Z ammo).
I have utilized head space guages to assess the chamber specs of many Ranger service rifles now. Perhaps I have even checked your rifle. Bolts should NOT close on a field guage. This is an automatic fail and an indicator of excessive headspace. If your rifle closes on a milspec field guage +.074" then have the rifle returned to headquarters. It likely just needs one or two bolt head sizes increased but it is best to have the rifle returned to HQ.
Additionally if the bolt will not close on a 'Go-Guage' your chamber may be a little to tight (or short). This is normally an issue with semi-automatic rifles (and floating firing pins) as it may result in what is known as a 'Slam-Fire'. Under normal circumstances this may not necessarily be a problem with a Lee Enfield but if you start including moisture or other types of debris in the chamber area you may experience an issue with effective bolt cycling. This could pose a safety issue if the rifle is discharged with the bolt only partially closed. Enfields were designed to be a bit on the loose side.
You use the weapon more than anyone else so pay attention to the indicators. Examine your brass occaisonally and look for any abnormalities. Look for chamber bulges, gouges, fire cracks in the case head area, split necks or other irregularities. Pierced primers could indicate a very short chamber or a firing pin issue. Flattened or cratered primers are indicators of high pressure. Know the indicators of headspace and chamber pressure and report any information that give you cause for concern. After all, these rifle platforms are 70+ years old and some of them may have surpassed it's designed life expectancy. Be safe...