Tech Tips

Headspace and Accuracy

by SingleShotLover


Most hand loaders know that an important determining factor in their quest for accuracy is consistent cartridge head space. Head space is regulated in several ways depending on the cartridge design.
 
The five cartridge styles common to American design are:
  • Rimmed - Most straight sided cases such as .44 Magnum, 45/70 and 22 LR and bottle-necked designs such as the .218 Bee and 30/30 Winchester
  • Semi-rimmed - .220 Swift for example
  • Rebated - .284 Winchester and .41 Action Express
  • Rimless - Miss-named, but examples are 30/06, .270 Winchester, .223 etc.
  • Belted - Raised "belt" incorporated into the case design at the rear-most portion of the case body as in .7MM Magnum, .300 Winchester Magnum etc.

Let's look at how each of these styles establish head space within a chamber:

Rimmed cases are designed to head space on the rim of the cartridge. Designed for ease of extraction, this is not an aide to accuracy. Rim thicknesses vary enormously even within the same lot of brass and causes inconsistent head space. Additionally, a case supported primarily by the extreme rear will also have a difficult time establishing consistent, accurate bullet alignment.

Semi-Rimmed cases are designed to "split the difference" between rim and shoulder head spacing and can be quite accurate if the case closely matches the chamber. (More on this later.)

Rebated and Rimless cartridges are both designed to head space on the case's shoulder (or in the .45 ACP and similar cartridges, on the case mouth) and provide for optimal bullet/bore alignment.

Belted cases primarily head space on the belt and are subject to the short-comings of the rimmed cases listed above.

How do we improve head-space consistency?

The main thing is to understand the design of your cartridge and plan accordingly. There isn't much that you can do to improve factory cartridge case/chamber fit, but the hand loader can improve this fit enormously with a bit of care.

Chamber match with rimmed, straight-wall revolver cartridges can be enhanced by choosing bullet diameters that are a minimal "slip" fit in the cylinder's throat. In the case of .44's, this could be anywhere from .429 to .431" diameter. This fit enables the entire case to be aligned more accurately with the chamber and (hopefully) the forcing cone of the barrel. Due to the action design limitations of lever actions and semi-autos, this method is not a practical consideration.

Head space with rimless straight-wall cartridges is determined by case mouth contact. For this reason, precise case trimming is essential and only taper crimps should be used.

For the balance of bottle-necked cartridges, whether rimmed, rebated, rimless, belted or semi-rimmed, head space should be regulated primarily by the case shoulder. This is easily accomplished by making sure that your sizing die is adjusted properly and doesn't re-size any more than necessary for reliable function. Basic loading instructions are intended to ensure that the loaded round will function in any suitable firearm...not necessarily for accuracy.

Starting with a lubed and "smoked" fired case, back your sizing die out several turns. Now by trial and error, turn the die back in by small stages until the point is reached where your press handle is bottomed out and inspection shows that the die has just "kissed" the shoulder of your case (and is minimally sizing the case body). Clean the case and try it in your action. you are looking for just a hint of resistance when you close the action. This is the shoulder of the case contacting the shoulder of the chamber. Once this stage has been reached, you are ready to lock your die in the new position.

STOP RIGHT THERE!

The coarse nature of the threads on loading dies are not accurate-alignment friendly. Since we have gone to all of the above trouble to try and make the case fit the chamber correctly, let's take one more step to insure optimal case/die alignment. With no case in the shell-holder, place enough washers (I keep a handful of "fender" washers handy) on the shell-holder to come into firm contact with the die base when the ram-arm is fully extended. This pressure helps align the die with the shell-holder. Holding this pressure firmly on the die, tighten the die's locking ring. (Do the same thing to your seating die for better bullet seating alignment.)

Remember that cases sized as above are only intended for the firearm from which they were originally fired. In the event of multiple firearms for the same caliber, it is best to have dedicated die sets for each. Minimal resizing also accomplishes a bonus to the hand loader; increased case life. By sizing no more than necessary you can reduce the stress to the brass and, in the case of cartridges reputed to be prone to head separation within just a few loadings (.35 Remington and .218 Bee come to mind) case life can indeed be increased.

Cartridges for lever actions, semi-autos and those for dangerous game may require more sizing for reliability. Certain belted cases may also require more sizing if the belt is not properly reduced by the above methods.

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