The "biggest" conforming cartridge should fit in the "smallest" conforming gun. This fit is controlled by headspace. We want the cartridge to fit sort of closely in the chamber of the gun, else accuracy may suffer and perhaps there is danger of powder gasses blowing back through the action.

     "Headspace" is a word with a number of slightly different meanings, all centering around the fit of the cartridge in the gun.

     Here's the SAAMI (Sporting Arms and Ammunition Manufacturers Institute) definition:

HEADSPACE: The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats.

     "Headspace " is sometimes used to describe the distance from the closed breech of the gun to the head of the cartridge, a few thousandths of an inch.

     "Headspace" is also used to describe the dimension from a datum point on the cartridge to the base of the cartridge, as measured with a cartridge headspace gage. Sometimes known as "cartridge headspace"

     "Headspace" takes on the guise of a verb: "The 30/30 cartridge HEADSPACES on the rim."

     Headspace is the distance from the breech face of the gun to either:

• the forward surface of the rim in rimmed cartridges, or

• the forward surface of the belt, in belted cartridges, or

• the case mouth, in straight cartridges, or

• a point on the shoulder, in shouldered cartridges.

     Other rimmed cases include the 38 Special, 357 Magnum, 44 Magnum, 30/40 Krag, 303 British and 45/70.

     Other belted cases include the 300 H&H Magnum, 7MM Remington Magnum and 375 H&H Magnum.

     Other straight cases head-spacing on the case mouth include the 45ACP, 9MM Parabellum, 32 ACP and 380 ACP.

Other cases head-spacing on the shoulder include the 308 Winchester, 243 Winchester, 257 Roberts, 8 MM Mauser and 35 Whelan.

     The headspace of a gun is measured with headspace gauges which come in sets of two (GO and NO GO) or sets of three (GO, FIELD, NO GO).

     Headspace of fired cartridges that headspace on the shoulder is measured with a cartridge headspace gauge. I think that what we're doing here is measuring the fired case to get an idea of how the gun chamber is dimensioned.

     Headspace of a gun-cartridge combination can be measured easily and cheaply by placing different thicknesses of shims, paper or ?, on the breech face, chambering the cartridge and feeling the force required to close the action.

     As an example, the headspace of a Springfield 1903 rifle can be checked/measured as follows:

• 1) Remove the firing pin.

• 2) Chamber a once-fired-with-a-full-load cartridge, noting the force required to close the bolt. A cartridge fired with a full load will expand to fit the chamber.

• 3) Measure the thickness of a piece of paper and put a small piece, covering the bolt face, on the bolt face.

• 4) Chamber the cartridge, noting the force required to close the bolt. Add another piece of paper to the bolt face.

• 5) Chamber the cartridge, noting the force required to close the bolt.

     Try combinations of onion skin, aluminum foil, different kinds of paper and cellophane until a combination of shims makes it impossible to close the bolt using reasonable force. You can always close the bolt, we're talking about reasonable force. I close the bolt with my thumb only, and can easily tell the difference between one and two pieces of paper for shims.

• 6) Measure your shim stack, that's the headspace. You might want to try this several times with that stack.

     The paper I use with my computer is ~.004", and if I can't close the bolt on two thicknesses of this paper, I think the headspace is O.K. If I CAN close the bolt on two thicknesses of this paper, it is time to have a qualified gunsmith look at the rifle.

     I can measure the headspace of Contender pistols (remove the extractor), rolling block and falling block and Martini rifles, and some auto-loading pistols by fiddling with shims and the gun and a new cartridge.

     I have had trouble with Remington rifles because I don't have the gizmo to remove the firing pin. I think that you need to have the firing pin out of the gun to be safe and "feel" the action close. 

     When the headspace is .008" or more, there may be trouble.

     Some guns that headspace on a belt or rim will show excess headspace with a new cartridge, but this excess headspace goes away after the cartridge is shot. I think that cartridges like the 30/30 in Contenders and single shot rifles blows out and thereafter headspaces on the shoulder or case taper. One of the causes of short case life due to splits and head separations is full length resizing of such cases, forcing them to undergo excessive work hardening.

     "Gunsmiths and factory machinists are human. They can get tired and careless. I’ve seen factory rifles with a 7x57 chamber and bore, but grooves for a .308 bullet. It sometimes happens that chambers are mis-cut and end up deeper - sometimes a LOT deeper - than they are supposed to be. If the case is supposed to headspace on the case mouth or the shoulder, this can result in dangerously excessive headspace. But oddly enough, such rifles are used every day, and work fine. It seems that in such instances, the case is prevented from going too deeply into the oversized chamber by the grip of the extractor. In effect, the round effectively headspaces on the extractor instead of the shoulder or case mouth as it was designed to do. The same thing is sometimes seen with auto-loading pistols and rifles, and other designs where the extractor engages before the case is chambered. It is not uncommon, even with classics like the M98 Mauser. There’s nothing particularly wrong with it, but you need to remove the extractor (and firing pin) if you want to measure ‘real’ headspace in your gun." Ken Mollohan

     "To check the headspace clearance on a bolt action rifle, I use this method:

• 1)  use a new case or completely pull down a factory loaded round so that the case may be used.

• 2)  start new primer into case about 1/2 way.

• 3)  chamber the case by hand.

• 4) close the bolt gently (this will not detonate the primer).

• 5)  extract the case carefully.

• 6)  with a depth gauge measure the amount of primer protrusion.

I have used this method for years with satisfaction.

     This method of headspace clearance measurement is not as good as using a headspace gage because of manufacturing tolerances of the brass, but it will show you any gross dimensional problems and is certainly close enough to demonstrate the benefits of fire formed cases." Norm Johnson

Shoulder Setback With Reduced Cast Bullet Loads.

     Sometimes cases that headspace on the shoulder will move the shoulder back and develop excess cartridge headspace as they are shot with light cast bullet loads. I have had this happen. I've had 30/06 cases where the shoulder moved back far enough the cartridge went in the chamber so deeply that I started getting misfires. I can't find my records on this, but I remember that it happened when I was firing light loads in 30/06 with cast bullets, attempting to learn how to shoot offhand. The wizards suggested I go up to 12? grains of Unique, I did and it solved the problem. This in an Engels & Trolsch M98 target rifle.

     Thanks to Paul Brasky for asking why we care about this-I forgot to write it down.

     If you're shooting light loads and the headspace increases because the shoulder sets back, you may get misfires and possibly accuracy suffers. This is a minor inconvenience.

     Of more importance is the possibility that you might reload some of these excess-cartridge-headspace cartridge cases with full loads. Then the excess headspace might lead to a case head separation and damage to the rifle and the shooter.

     There are three theories on how the shoulder is set back:

     Some claim that the firing pin fall drives the case forward, driving the shoulder back and increasing headspace. Then when the cartridge fires the case expands, gripping the chamber. This leaves the case with the shoulder pushed back, and excess headspace.

     I have twice fired one case with ~20 oiled - new dead primers in rifles with the extractor removed, and have not yet had the headspace increase. The firing pin didn't drive the case forward.

     On 11/16/06, I did this test for the third time on the third gun, a Win M70 Target 30/06 Post 64 rifle. I "killed" 26 CCI 250 Magnum primers with marvel mystery oil. (I did this test at home, in the condo.)

• Put an empty fired case in the gun.

• Removed the firing pin from the bolt.

• The bolt closed easily on the case.

     Made a .004" copier paper patch to fit the bolt face, put it on the bolt face, the bolt would not close with a little pressure.

     After testing and fiddling and firing 26 primers in that one case, the bolt closed easily on the fired case-wouldn't close with a little pressure with the paper patch on the bolt face.

     Others, including Tom Gray, have increased headspace with the firing pin striking the primer, but I can't and this is the third time I've done this test.

     Some claim that the primer explosion pushes the case forward, driving the shoulder back. The primer and primer pocket act as a piston and cylinder, with the primer gas acting on the forward surface of the primer pocket, around the flash hole. These folks tell us that the problem can be solved by drilling out the flash holes to a larger diameter, and keeping these cases for light loads only.

     On 18 Nov., 2006 I took the Win M70 30/06 Post 64 rifle and taped a 2 liter Pepsi bottle on the muzzle to hold the noise down.

     (TRK points out that taping a Pepsi bottle to the muzzle of a rifle is against the law, that the BATFE considers this "making a silencer" So don't do it, or you could end up in jail. Or in hell. If any BATFE guy is reading this, it's all a lie, I never did it. Don't drink Pepsi.. Had no tape.)

     Put an empty fired case in the gun.

     Removed the firing pin from the bolt.

     The bolt closed easily on the case.

     Made a .004" copier paper patch to fit the bolt face, put it on the bolt face, the bolt would not close with a little pressure.

     Firing pin in , CCI 250 Magnum primer in the pocket, in the gun, fired, a little pop.

     Tried again with the paper shim, no firing pin in the bolt. The bolt closed with no resistance. The primer drove the case forward.

     Fired four more primers, made another .004" shim, both shims on the bolt face and the bolt closed on the (primer-less) case with a little resistance.

     In my guns, with my tests, it's the primer driving the case forward, not the firing pin.

     Mike in CO was kind enough to give me a reference to "Handloading", NRA, 1981, William C. Davis Jr.

     After the testing, another reader scanned and sent "Reduced Loads", pages 113-115, by Mr. Davis. On page 114 he states clearly that it is the primer firing that moves the case forward and moves the shoulder back.  My experiments agree with this.

     Drilling the flash hole out (#28 drill) fixed the problem for some folks. Here's why, I think.

     A large primer pocket is .210" diameter, area is .035"^2 (square inches).

     Standard flash hole diameter is .080", area is .005"^2.

     #28 drill diameter is .1405", area is .016"^2.

     With a standard flash hole the primer acts on .035-.005 = .030"^2.

     With the flash hole drilled #28, the primer acts on .035-.016  = .019"^2.

     This change, we're told, keeps the primer from driving the case forward, the shoulder back, and increasing headspace.

     "I shoot literally thousands of Lee TL314-90-SWCs, Hornady .314 swaged lead, Speer .32 cal HBWCs over 2.7 to 3 gr of Bullseye in rimless case; 7.63x39, .308, 30-06 and 7.65 Argentine. I also have shot them in .30 Rem, .300 Savage and 7.7 Jap (probably a couple more I'm not remembering). In ALL instances, even with well fire formed cases, headspace of the case decreased consistently with each firing. Misfires were most often not encountered with controlled feed actions. However with push feed actions misfires occur in as little as 3-5 firings. In every instance drilling of the flash hole stopped the shoulder set back.

     The exception is 7.62x39 with R-P cases using SR primers. I've yet to experience shoulder set back with the load mentioned even after 50 firings in a bolt action Mini Mk X rifle. But switch to IMI or W-W cases using LR Primers and shoulder set back occurs just like other rimless cases using a LR primer. "Larry Gibson

     "I have also experienced this shoulder setback with light loads in rimless cases  My experience is also that the primer pressure is the primary - but not the only - culprit: I have seen this only in situations where the round and chamber dimensions are such that the round chambers very freely, and there is some freebore between the bullet ogive and the chamber leade.

     It seems that substantial pressure is developed within the primer pocket, which drives the case forward and the primer backwards. The case is stopped by impacting the chamber shoulder, while the primer is stopped by impacting the breech face. Metal deformation results from both impacts: The shoulder of the case is swaged back slightly, and the primer is slightly flattened against the breech.

     Now pressure from combustion of the gunpowder begins to develop: In ‘normal’ rounds, this pressure pushes the case head back over the primer, re-seating it in the primer pocket, and reverses the shoulder setback just like blowing up a balloon. Note that if the sides of the case do not have some slip against the chamber walls, the process of pushing the case head back will necessarily result in stretching and thinning the brass. This in turn leads to case head separations and the need to trim cases occasionally.

     However, many low pressure loads do not generate sufficient force to push the case head backwards over the primer. As a result, the shoulder swaging is not reversed either, and the primer can be seen and felt to project slightly above the head of the case. The amount of the primer protrusion is equal to the effective headspace in the rifle / case combination. (Note that many combinations actually headspace on the extractor grip on the groove, not on the case shoulder as is usually assumed.) Shoulder setback is easily prevented and or remedied by simply seating bullets out into firm contact with the leade. In this situation the round effectively headspaces on the bullet ogive, not the extractor groove or the shoulder. The case is effectively immobilized between the bullet and the breech, with zero headspace. The primer cannot move backwards, and the case - as a unit - cannot move forwards. But the case shoulder is not in contact with the chamber shoulder. Internal pressure of a moderate load can expand the case shoulder to fill the chamber. This will correct any headspace issues with that case and rifle combination."  Kenneth Mollohan

     Some say that this shoulder setback problem can be cured by leaving a little sizing lube on the outside of the case. I've never tried this.

     "I use Imperial Sizing die wax and size just below the base of the gas check in my match loads. Pressure is less than 24,000 CUP according to the load manuals. I have had problems with the headspace reading of the case decreasing upon firing with full length resizing and Winchester Large Rifle primers. This was with a carefully cleaned and dried chamber and case.

     Now I leave the case lube on and the headspace stays consistent. My theory is that there is enough internal pressure to keep the case pushed back against the bolt face. This appears to have lessened flyers from my groups." Ric Bowman

     "Why remove the wax-based sizing lube? I know, the liquid lube folks always say to remove their product before reloading. However, here is my take on this issue: I avoid liquid lubes and then leave the thin wax-lube coating alone.

     Why remove wax?! I use Lee Sizing Lube, which is a water/wax emulsion. It takes amazingly little to make the brass just fly through that die, then I just load it. It is not a semi-liquid that can build up in the chamber; rather, it dries into a very thin coating that is integral with the case. Case head thrust? That's no reason. Hell, our rifles are designed to handle that. High pressure? If you play with QuickLOAD, you'll find that your favorite target loads are, mostly, in the 5-15 Kpsi range and most listed hunting loads are in the 10-20 Kpsi range. Noooo praaaaab-lem.

     The thing that makes the Lee stuff work best is to size right after putting the stuff on (i.e., don't let the water evaporate out of the emulsion prior to sizing the case). You can buy it by the case from Midway. If you do, do NOT store it out in the garage, unless you live in Florida, given that freezing temperature makes it tend to separate a bit. Keep them stored in the house. Soooo, I put on only as much as necessary (veeeeery little), size the case, then skip the wipe-off step. I figure that there are other more important things to focus on, as follows.

     One trick that I just learned from Mic McPherson's excellent book Accurizing The Factory Rifle (available from Precision Shooting Magazine) is that one should either have the sizer die backed off to just get the first 1/4" or so of the case neck or should have it come down hard on the shellholder. The problem is that these are the only two setups that guarantee a uniform headspace (no change, for the first one, or the same headspace on each case, for the latter).

     If the case shoulder is partially sized back, the springiness of the case, die, and press will cause the ACTUAL headspace to vary as much as 0.020", from case to case. Redding sells "high" shellholders in steps of from +0.002" to 0.010", allowing one to eliminate over-sizing (from a too-short die). Mic uses a little tape trick (described in the book) to determine which such high-holder is just riot for a given rifle/sizer-die combo." Ed Wosika

     There's a third theory on this shoulder setback; that neck sizing in a rough sizing die may set the shoulder back. See below and also Ed Wosika's comments above.

     NV Curmudgeon on Cast Boolits writes: "Let me tell you what I found by empirical means. I have a re-barreled Ruger 77 in .35 Whelen. There is never a bit of case fit trouble when firing full power loads. I use full power cases until I am bored with them. Using my standard CB load of 20.0 X 2400 and a 213 gr. (actual weight) NEI 220358 boolit at a mv of 1700 fps, and PARTIAL FULL-LENGTH SIZING, I begin having misfires around the tenth firing. This occurs with either Rem. .35 Whelen cases or cases formed from Rem. .270s. The Wilson case gauge then shows a reduction in head to datum line measurement. I next bought a Redding (real) neck sizing die. So far the group of cases that I am watching are still sure-fire after 17 firings. My wild guess is that the partial full-length sized, low pressure loaded, cases are not obturating as well as desired, and that the P.F.L.S. is exacerbating the situation by very gradually setting back the shoulder."