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GUNS Magazine January 2013 Digital Edition - Page 28
let’S lOOk inSide cAse sizing pARt ii Glen Zediker ast time, we talked about resizing a spent cartridge case for best reuse in an AR-type firearm. The focus was on the outside of the case. This time we’ll talk about the inside, the case neck area. Options matter more than you might imagine. Before talking options, the first step is understanding the essence. There are two diameters on a case neck, both matter. Inside diameter and outside diameter. On firing, the case neck will expand to fit the limits of that area in the rifle chamber. For safety’s sake, most factory chamber neck areas are fairly generous in size (compared to what we might get away with in a custom chamber and consistent brass). That is done to accommodate a wider variety of cartridge case specifications. Not all case neck walls are the same thickness, so not all neck diameters will be the same. There has to be room for the case neck to expand to release the bullet. If there’s not, then pressures can escalate. In resizing, the case neck gets its outside squeezed down and then its inside gets opened back up. L Most conventional sizing dies are going to take the fired neck outside diameter down a significant amount. These dies incorporate an expanding appliance, usually called a sizing button (some, me too, call it an “expander”) that comes back through the case neck when the case is withdrawn from the die. This is affixed to the decapping rod. The button diameter determines the inside case neck diameter, and also then the outside neck diameter. It’s inside diameter that’s important. Inside diameter determines case neck constriction, which some call case neck “tension,” and that matters a whole lot. It matters to safety and accuracy. It’s the difference in diameters of the bullet and the inside case neck. For a semi-auto it should be at least .003". Less than that and retaining the bullet against movement prior to firing can be a question. This movement can be induced through inertia or impact. Much more than that (more than .005" difference) and then the bullet may get its jacket damaged on seating, as well as having the bullet become an unwanted contribution to the sizing operation. If there is excessive seating resistance, the case shoulder may get additional setback. Some, me included, are concerned with the amount of down and up in the sizing operation. No doubt, more sizing “works” brass and shortens its life. There are sizing dies that feature changeable bushings to specify the amount of outside case neck sizing. It’s Exaggerated, of possible, certainly, course, but because to use this to reduce wall thicknesses the amount the vary there literally can be two different expander opens up centers in a case the neck. neck with nonI really don’t uniformed walls. recommend bushWhich influences depends on whether ing dies for semithe case neck was autos. One of the last sized inside or main points against outside. If you don’t them is that they neck-turn cases, it should be inside. don’t size the full height of the case neck. Not sizing the full neck tube is a contributor to the influence of the case neck “doughnut” I talked about in the October 2008 issue. (In case you missed that, this is narrow elevated ring of brass that increases constriction by reducing the inside neck diameter. It’s like rolling an O-ring down into the case neck, stopping it right at the case neck, case shoulder juncture.) As I hope makes sense, it’s also for this reason I don’t recommend sizing a case without a sizing button in place. This is easily possible with a bushing die. Squeezing down the outside diameter of the neck without opening its inside back up will, I promise, form a doughnut. The sizing button reopens the neck inside and helps alleviate the effects of this condition. It can also influence accuracy, in a bad way, contrary to the intentions of following this procedure. neCk Centers This WW-brand case has .012"-thick neck walls. Double that to get .024". Add that to the bullet diameter, .224", to get the outside case neck diameter of a loaded round. Of course you can always just measure a loaded neck, but this progression of attaining numbers shows more. For example, take that figure, .248", and reduce it until you get the .003" recommended constriction amount. That means we need a ready-to-load outside diameter of .245", which would be an inside diameter of .221". If using a neck-bushing die, it would be a .244", or a sizing button diameter of .222", both account for the .001" spring-back. (Always either add or subtract .001" from any sizing appliance to arrive at an anticipated net result. Brass isn’t completely pliable, or plastic. It will rebound roughly this amount after any sizing operation.) In effect, there are two centers on a case neck, one outside and one inside. The inside matters most because that’s where the bullet is, and that’s what it gets seated into. To see how literal this is, sizing a case with no expanding appliance, such that only the outside wall is touched, and then running it on a concentricity fixture will almost always show zero to very little runout. 28 W W W. G U N S M AG A Z I N E . C O M • JA N UA RY 2 0 1 3