Reloading the 303 British: Chambers and Headspace

copyright 1999 – Stephen Redgwell

Reloaders who are new to the 303 British cartridge have to understand that their chambers are probably a little long – NOT larger in diameter. Unlike today’s commercial firearms, the chamber dimensions of most Lee Enfields vary greatly in size. It’s this internal space that must be considered when reloading for the cartridge.


Lee Enfields were built in many countries, during peacetime and while at war, for over sixty years. Production methods and quality controls differed greatly. Some saw severe service while others went virtually unused, spending years on armourer’s racks or in storage containers. Is any of this really important? Oh yes! Being aware of these variables and adjusting for them will produce tighter groups and extend case life. What makes them different?

1. They were built for military use and not for commercial sale.
2. Many have seen war service and have been bounced all over the world.
3. Some were well maintained and properly stored while others were abused and ignored.
4. No thought was given to reclaiming and reloading fired cases.
5. Each rifle is a law unto itself – more so than virtually any other firearm – commercial or not.


Most modern rifle cartridges are referred to as rimless, but that’s not 100 percent correct. They all have a rim – it just doesn’t extend past the wall of the case. Rimless cases use the distance measured from the bolt face to the middle of the shoulder to determine headspace. This length or measurement is standard. It has been predetermined and accepted by the industry as a given length.

But what about the 303 British? It’s a rimmed case!

Looking at the picture above, you can see a shaded area just forward of the bolt face. This is a recessed area that houses the rim when a cartridge is chambered. The size of this area is measured from the bolt face to the end of the recess, when the bolt is closed. How large this area is, determines headspace. If this distance increases, your headspace is moving toward excessive.

There are two standards I know of for measuring headspace – CDN military and SAAMI/Australian.

GaugeCDN MilitarySAAMI/Australian
NO GO.070.067

Most gunsmiths use the tighter SAAMI gauges when accessing headspace. I said most – not all. If your rifle is assessed as “serviceable” using military gauges, there is a distinct possibility that you will experience more case stretching. If this is so, you might want to tighten up the headspace.

A THIRD STANDARD?? – Information provided by David Moses – Thanks!

There is one more standard for the .303 and that is the CIP norm. CIP (see explanation below) states that the max cartridge rim thickness R is 1.63 mm; min barrel relief R is also 1.63 mm. The max allowable headspace (in the case of the CIP, this is the air space between the max ctg R and the breech face) is 0.15 mm.

In our terminology, “go” would be 1.63 mm or 0.064″; “no go” would be 1.63 + 0.15 mm = 1.78 mm or 0.070″. This is obviously based on military specs. Naturally, there is no “field” gage for CIP proof norms.


If you’ve got a No 4 rifle, check the bolt head. On the top, there is a number stamped into the metal. The numbers go from “0” to “3”. Each increment of one adds 3 thousandths of an inch to the length of the bolt head. If the number stamped there is 0, 1 or 2 you will be able to adjust the headspace by simply removing (unscrewing) the bolt head and installing another with the next highest number. It will be necessary for you to check using the gauges again.

If you’ve got a No 1 rifle, the fix isn’t as easy. Leave it with the gunsmith and let him work his magic. The bolt heads are not interchangeable like the No 4s.


So, you chamber the cartridge and lock it into place by fully closing the bolt. When you fire the cartridge, the sides, shoulder and neck all expand outward to fit the chamber wall. They do spring back slightly, but what you have, in effect, is a tailor made case that is unique within the world of Lee Enfields. Most people notice that the shoulder moves forward when they hold a fired case up beside an unfired one. Remember, shoulder position is not important for headspacing rimmed cases!

This is where many people start causing the trouble of cracks and separations. They take this marvelously formed case, which, like a snowflake, is one of a kind, and run it through their full length die…crushing it back to its approximate, original size.

Subsequent firing and full length resizing overworks the brass and its life is reduced. It’s this complete cycle that prematurely ends its usefulness. The solution is simple – neck resize!


To stop overworking the brass, neck resize your cases. The major benefit comes as more shots from each case. It may also improve accuracy. Less effort is required to neck size, and it is easier and faster. Some die companies have optional elliptical expanders, which work the brass a little less as well. Every little bit helps.

Case stretch is inevitable – so when should you full length resize? When the shoulder moves too far forward and chambering becomes difficult. You have a couple of options. The first is to push the shoulder back by adjusting your full length die to resize the neck and just touch or “bump” the case shoulder. Using this method will also partially reduce the case body.

The second option is to use a special body die made by Redding. It’s designed to full length resize the body and move the shoulder back without reducing the neck. It’s an inexpensive addition to your bench that will help resize cases whose shoulders have moved forward with use.

Let’s review the steps to extend the life of your brass.


1. Tighten up your headspace according to SAAMI specifications
2. Fireform cases using midrange loads
3. Neck size only
4. Use an elliptical expander in your reloading die
5. When chambering becomes difficult because of shoulder position,partial resize with a full length die or use a Redding Body Die
6. Don’t use max loads unless you have to!
What is CIP?

Adapted from Wikipedia – The Commission Internationale Permanente pour l’Epreuve des Armes à Feu Portatives (Permanent International Commission for portable firearms testing) – commonly abbreviated as C.I.P. or CIP – is a European ammunition standards organization.

CIP safeguards that every civilian firearm with a CIP registered chambering (generally all non-wildcat chamberings) gets professionally proofed at a CIP certified test facility, before it can be sold to consumers in a CIP regulated country.

The standard proof test consists of firing an overpressure cartridge that produces 25% more chamber pressure then the pending CIP specified maximum chamber gas pressure limit for that particular cartridge. Voluntarily testing beyond the currently (2007) legally required 125% standard test benchmark is often also possible for consumers who intend the use their firearms under extreme conditions (hot climates, long strings of shots, etc.). A pass mark is stamped in every successfully tested firearm.

CIP also publishes various industry standards such as ammunition and chamber dimension specifications, maximum allowed chamber pressures, cartridge/calibre nomenclature, etc. that have indisputable legal status in CIP member states.

Only the military organizations and other firearms bearing government agencies from the CIP member states do not legally have to comply with CIP rulings in CIP member states.