Use Chamber Concentricity to Fit Ammo to Your Rifle

There are many different ways to headspace bottlenecked cartridges, from the rim, which is popular with lever-action cartridges, to the Holland & Holland design that uses the shoulder, and even the true rimless cartridges. Proper chamber concentricity can improve accuracy and make a shooter smile. To understand the situation, it is important to realize that not every chamber is cut the same way. Even though our factory guns, and even custom work, do their best to keep everything uniform, even with modern CNC machining, precision gauges, and computer-designed chambers, they might not all be cut the same way. Even though I believe that our mass-produced ammo is the best ever, there will be slight variations between brands and lots. He or she may sight the rifle in advance of the season and take a few shots, to ensure that they can hit a paperplate (roughly the same size as a deer’s vital organs), or even a little better. This is fine if this is your hunting style. For someone who is serious about rifle competitions or target shooting, or is preparing for a long-range hunt, a different level of precision and mindset is required. The pressures created by the firing sequence have expanded the cartridge to the dimensions of its chamber. The difference between the original SAAMI dimension and the fired (chamber dimensions) is what allows the cartridge to be inserted into the chamber easily, but that same difference also reduces the chamber concentricity. Belted cases are known to have case head separation, especially when they are brought back to SAAMI dimensions after each firing. Belted cases will stretch the headspace to the point where the case will separate. This phenomenon can be drastically reduced by leaving the shoulder in the same position it was when the case was fired. Photo: Massaro Media Group. If we’re discussing a rimless casing, where the shoulders handle the headspacing it’s easy to see how accuracy could suffer when the ammo isn’t matched up well with the chamber. Reloaders have taken steps for years to keep the cases as close as possible to the chamber conformation. The neck sizing method was popular in the beginning. The neck of a cartridge can be “neck-sized” to reduce the tension needed to hold a projectile. The shoulder position and the body diameter of the fired casing are left “as-is.” This technique is effective, but it has some pitfalls. Neck-sized ammunition is generally only suitable for bolt action rifles that can close the bolt with ammunition of greater dimensions. RCBS neck-sizing dies are a great way to neck size cases without overstretching brass. Photo: Massaro Media Group. Also, neck-sized brass is usually reserved for the rifle that fired it. The slight variations in chamber sizes can prevent neck-sized case from being used in another rifle, as the bolt may not close at all. Even when properly fed in a bolt action rifle, the feeding can be less smooth. This is especially true when hunting dangerous game. Most manufacturers of reloading tools still offer a neck-sizing die. The other way to achieve the best chamber concentricity and still have smooth and rapid feed is to measure the shoulder movement between the SAAMI specification and your fired cases. Redding’s Instant Indicator & Bullet Comparator can be used to measure how much your cases stretch when fired. Using a SAAMI spec dummy as a benchmark, the Comparator shows you how far your fired cases differ from the specified dimensions. It can be used with the Redding Competition Shellholder Set to shrink the case’s exterior dimensions without moving the shoulder. If you compare a half dozen of your fired cases with the SAAMI dummy and see a difference of only 0.006 inches, you can grab the shellholder that corresponds to this size. Redding’s set of shellholders offers five different depths for each family of cases, ranging from 0.002 inch to 0.010 inches. You might need to use a shoulder size that is 0.01 inch smaller than the observed variation. But it will still improve accuracy. Cartridges loaded this way will have the same feeding benefits as a fully resized cartridge, but will also best match your rifle chamber because the shoulder will be up against the wall. These cartridges are rifle-specific in that they should only be used in the rifle that they were originally fired. However, the chamber concentricity and accuracy will improve. They have certainly extended the life span of my.300 Winchester Magnum, and.300 H&H Magnum Brass. When I run into a problematic rifle, I use the silly shellholder trick. It has worked in several cases where a seemingly lame cartridge has shone once the shoulder is properly mated with the chamber. You can easily customize your cartridges to match the dimensions of the rifle’s chamber. This will result in a cartridge that is easy to feed and has the best concentricity. I’m confident that you will see an improvement in accuracy and consistency of velocity. This is what we reloaders are most interested in. These targets are designed for accuracy, especially long-range shots. These targets are perfect for handgun and rifle enthusiasts. They are designed to be accurate, especially for long range shots.