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GUNS Magazine June 2010 - Page 14
GUNSMITHING • HAMILTON BOWEN • HEADSPACE We all have it. Y ou can’t hang around gun nuts, gun cranks or gun junkies very long without hearing the term “headspace.” All guns have headspace just like all people have blood pressure. And your gun can have excess (or even insufficient) headspace just like people can have high (or even low) blood pressure. Put simply, headspace is cartridge endfloat in the gun’s chamber. Put another way, headspace is the amount of movement back and forth a cartridge has along the chamber axis. Good headspace is the optimal clearance as defined by SAAMI (Sporting Arms and Ammunition Manufacturers Institute), an organization founded in 1926 at the behest of the federal government (relax—they got this one right) to promote industry standards and specifications of safety, uniformity, interchangeability, etc. The European equivalent is the Commission International Permanente Pour L’Epreuve des Armes a Feu Portatives or, en Anglais, Permanent International Commission for Firearms Testing, commonly abbreviated as CIP. Of immediate interest to us is the standardization of production cartridges and chamber specifications, which include headspace numbers. Proper headspace is a balance between clearance needed for function on the one hand and cartridge integrity on the other. Headspace is defined in terms of the chamber specification, not the cartridge specification, but improperly formed cartridges are a source of functional headspace problems just the same. For the best education in the matter, there is no substitute for a set of SAAMI Mostautoloadingpistolcartridgecasesare rimlesscasesandheadspaceoffthecase mouthratherthantheshoulder,aswoulda rimlessriflecartridge. Thegreat.300Holland&Hollandcartridge,an earlybeltedmagnumround,doesn’thavemuch ofashoulder.Thebelted-headdesigngaveboth excellentfeedingandheadspacecontrol. Thislovelypre-warSavageM99takedownin.30-30bearsaheadspacecheckbeforegoingafield. 14 cartridge/chamber drawings. My aging NRA Handloader’s Guide has a fairly complete set. Like lots of things, there are only two forms of bad headspace: too little and too much. Too little headspace is about as rare as a housecat without halitosis. In this instance, the chamber is too short and a standard-spec cartridge may prevent the action from closing, at least without a “crush” or interference fit. In a typical magazine rifle the bolt won’t close without pressure, if at all. In revolvers, the cylinder won’t close or rotate if the gun is loadable at all. Excess headspace is the usual culprit and can be potentially dangerous in extreme cases. In my experience, there are a couple of typical manifestations of headspace. Too much can lead to erratic ignition or out and out misfires since the cartridge will be pushed forward to the end of the chamber by the firing pin before it can do its job, which both robs the pin of real impact energy and puts the primer anvil further out of reach. In some guns, this condition may go unnoticed since the extractor hook may keep the cartridge near enough to the firing pin to achieve ignition. More worrisome is where the case body stretches on firing. If the case is repeatedly trimmed and reloaded, at some point, on firing, the head will separate from the rest of the case. In extreme cases, a new cartridge fails completely. In either case, depending on the gun type and design, high-pressure gas can leak through the action with disastrous results. We’re talking about really high pressure here. Even the lowly .22 LR generates 20,000+ psi. Magnum rifle and revolver rounds can exceed 60,000 psi. Your car tire works at about 35 to 40 psi. Stocks and grips splinter. Action bits and pieces fly off. If you are stupid enough to shoot without eye protection, you could get issued a white cane. Catastrophic gun failures due to extreme headspace are practically unknown but could occur where extended shooting batters action components to the point they let go, assuming the gun and shooter aren’t WWW.GUNSMAGAZINE.COM • JUNE 2010