Hi folks, Eric here
my 2nd post is perhaps the most important to hand-loading quality ammunition. I load for a bolt action rifle so my observations relate to them, Autos and pumps require different tolerances. Also I will endorse some products I like to use and make no money for my endorsement.
Head-space and cartridge overall length (COAL) are related and involve the same measuring tools.
Lets start with Head-space.
Head-space is a complex issue but for introduction, it is the portion of the loaded cartridge that determines the depth in relation to the bolt face that the cartridge enters the chamber. For example, Belted magnums are designed to “head-space” on the belt, meaning the cartridge proceeds into the chamber no further than the belt will allow. Most non belted cartridges head-space on the shoulder of the case. This means that the cartridge bears on the shoulder when the bolt is closed. When the firing pin strikes the primer the cartridge does not move forward because the shoulder is touching the chamber wall. Rimmed cases (mostly straight walled cases) don't move forward on the primer strike since the rim is bearing on the chamber mouth.
OK, most of the non magnum cartridges that we are interested in are headspaceing on the shoulder of the cartridge, and even magnum re-loaders use the shoulder to head-space their cartridges since it is a better way to limit case stretching and maximize brass life and get the best accuracy.
I don't want to get bogged down in a discussion of the technical details of head-space but encourage you to explore the issue on your own.
What is important to our reloading discussion is MEASURING head-space. This is the fundamental measurement in adjusting your full length re-sizing dies. I don't plan to discuss Neck sizing cases here but will address that style of reloading in a future post.
To put it simply, my goal in re-sizing brass for reloading is to reform the spent cases to a dimension that either closely matches the chamber of the gun it was fired in or to bring the case dimensions within the SAAMI (Sporting Arms and Ammunition Manufacturers Institute) specs. For accuracy we will assume you are loading for a specific bolt gun and not interested in making generic ammo fitting all guns of the same caliber.
Most reloading sets describe setting up your sizing die in terms of shell holder contact with the die. This will probably get your ammo into the SAAMI specs but is not the way to get the best fit with your formed cartridge and the chamber of your rifle.
My way to size a fired brass shell is to measure the length difference between the base of a fired case and the point on a re-sized case where it contacts the chamber. The question is where to take the measurement, and if your cartridge head-spaces on the shoulder, then we need to measure the length from the shoulder to the base.
Cartridge brass expands on normal pressure firing and rebounds to a dimension smaller than the chamber (allowing easy extraction, if you perceive hard extraction you may have exceeded the pressure tolerances of the shell's brass).
The rebound of the brass in bottle neck cartridges is predictable and if we want to make consistent ammunition I try to re-size the brass to around 1 or 2 thousandths of an inch below the fired dimensions. I remind you that these dimensions are not precise and some loaders use closer tolerances. Neck only sizing, for example, allows for a theoretical better “fit” between the case and chamber, and if the chamber is perfectly true, can result in better performance regardless of the orientation of the reloaded cartridge in the chamber… if the chamber is slightly off, neck sized ammo can be less consistent than Full length sized since it needs to be chambered in the same orientation as it was fired (very difficult) and since we are trying for consistency and not a perfect match, full length sizing is our best chance of repeatable precision.
OK now we get to the measurement aspect of reloading. The single most important tool you as a re-loader must have is a reasonably accurate set of CALIPERS. I use a Mitutoyo digital caliper and recommend it to anyone but Starret and others work well. To measure head-space you will need a Comparitor tool that along with your caliper, properly sits on the shoulder of the cartridge you load for. The Stoney point/Hornady or similar Sinclair set, attaches to your caliper and allows you to get a repeatable measure of the distance from the base of the shell to a point on the shoulder of the cartridge. This measurement is our guide for determining the amount of adjustment to our full length sizing die. We simply measure the fired round with these tools and remeasure the re-sized shell and if it is 1 or 2 thousandths less we are good. In theory this measurement is the gold standard of reloading but I find that the caliper & comparitor require a high degree of technique and competence to get repeatable measurements. I personally cannot maintain better than 15 thousandths of an inch variation in measurements using these manual techniques. I now use the digital comparitor from Innovative Technologies http://www.larrywillis.com. The key to making this tool useful is locating the spot on the shoulder of the case that best shows the “bump” that represents the length we are looking for.
To make a long story short, size your brass to 1 or 2 thousandths of an inch short of your fired brass and continue on to the next step which for me is COAL or Cartridge Overall Length.
When we talk about COAL (Cartridge Over All Length) we will discuss using calipers to determine distance to lands and seating depth.
Happy loading and read those forums !!!!
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