Reloading: Use your Head

The subject of headspace first surfaced about a year ago. This month I’ll be looking at it in detail as to how it affects the reloader. Put simply, it’s the term used to describe the key dimensional parameters that largely ensure a safe fit of the cartridge in the chamber and the means by which it is achieved. Less simple is the fact that there are several fundamentally different cartridge designs, meaning that no single rule or definition could embrace this diversity. The result is therefore a suite of four headspace specifications.

However, we must not assume that the correct control of headspace guarantees your safety. Whilst correct headspace ensures that the right sized cartridge safely fits the right chamber it does not prevent some dimensional irregularities from creating dangerous pressures or stop the wrong cartridge from fitting the wrong chamber – vigilance is still required if accidents and injuries are to be avoided.

The Original ‘Head’ Space Cartridge

The simplest of these four cartridge configurations is the one that gave its name to the definition. Rimmed cartridges, such as the .22 Long Rifle, .38 Spl and .30-40 Krag are located in the chamber by their rim, or ‘head’. (FYI, bullets are NOT heads – they are projectiles). For this design of cartridge the head space is therefore the distance between the chamber face and the face of the bolt in an auto, or the recoil plate in most revolvers. Dimensionally it therefore amounts to the sum of the rim thickness and a suitable tolerance. This definition also applies to semi-rimmed cartridges such as the .380 Auto as it still has a rim diameter in excess of the body of the case.

Stretched To Breaking Point

So what does this specified headspace dimension achieve? Well, in this instance it ensures that a correctly manufactured cartridge will chamber in the firearm and should not suffer from case head separation when fired. Separation usually resulting from excessive stretching of the lower web of the case when fired. During this cycle the slim upper wall of the case is forced tight against the chamber whilst the rigid base web and head of the case moves rearward until it contacts the recoil shield or bolt/breech face.

However, the control of headspace does not prevent an over length cartridge from entering the chamber. In many revolvers the presence of a reduced diameter throat serves to limit chambered cartridge length as well as reducing blow-by and loss of gas pressure. Nonetheless, a roll crimped revolver cartridge loaded with an untrimmed, seriously over length case could still be chambered. It would then fail catastrophically when fired, due to the roll crimp being trapped by the throat step in the chamber. 

Rimless Parallel Wall Cartridges

The .45 ACP is a good example. Here the rim diameter is the same as, or less than that of the case body. Headspace is therefore specified as the distance from the breech/bolt face or recoil shield to the step at the end of the chamber and against which the neck of the case locates. It is immediately obvious that such cartridges cannot be produced with a roll crimp or excessive taper crimp as this would impair the neck location of the round. Slightly more complicated for hand loaders is the chambering of rimless tapered cartridges such as the 9mm Parabellum. Overly enthusiastic full length resizing of the 9mm is a common problem, causing the cartridge to be a poor fit in the chamber and, in extremis, to chamber the round on the bullet ogive against the rifling. In such instances the explosive separation of the case head is almost inevitable.

The Most ‘Standard’ Cartridge
Next we have the standard rimless bottleneck cartridge – the default design for most modern rifle ammunition. A good example being the .308 Winchester. Here headspace is defined by the distance from the head of the case against the closed bolt to the chamber contact point along the bottleneck taper – this point having a specified diameter. For our .308 Win the two dimensions are 1.640” Max/1.630” Min and 0.400” dia. This definition also applies to rebated bottleneck cartridges such as the .284 Win. Mag.

Lastly, the least common option - belted cartridges. For both parallel and bottleneck belted cartridges the headspace reference dimension is from the face of the closed bolt/breech face to the contact point of the front face of the belt. Examples are the .458 Win Mag and 7mm Rem Mag respectively.

Check It Out

The SAAMI pdf data sheets with dimensioned line drawings for rifle and pistol cartridges can be found at:-
www.saami.org/specifications_and_information/publications/index.cfm

Which brings us to two other vital statistics provided by SAAMI, COL or Cartridge Overall Length (sometimes referred to as OAL) and maximum case length. The maximum case length is a fixed value for every calibre and is used as a basis for the overall chamber length. The chamber length is set to give a safe clearance for a cartridge case of the correct length. However, as an untrimmed case grows with use there is the risk that the neck could enter the throat of the chamber thereby restricting its ability to release the bullet and greatly increasing pressures. Trimming cases will therefore be a future topic. Unlike case length, COL is not a fixed value for a given cartridge but rather a variable controlled by the design and seating depth of the bullet being used. Again, we’ll look at this in a future article. 

Firearm designers and cartridge manufacturers use sets of gauges to confirm the compliance of their products with these specifications. Visitors to the SAAMI (Sporting Arms and Ammunition Manufacturers’ Institute, Inc) can download files containing these data. Unfortunately the web site of the CIP (to which I referred in an earlier article) does not offer the same service. Commercially produced gauge sets for the most popular calibres are manufactured by Okie, Clymer and Forster amongst others. Standard gauges cost between £25 and £40.