You’ve selected the best brass you can find, and have deburred the flash holes. There’s one more step to complete before loading the cases – primer pocket uniforming. This is a crucial step when trying to create the most precise target loads possible because it minimizes any variations in firing-pin impact. Not uniforming the primer pockets can cause velocity variations that lead to vertical stringing. And considering that the 2013 Super Shoot benchrest championship was determined by only .0001", there’s no such thing as "overkill" in case preparation if you’re looking to compete with a chance to win.
To better understand exactly how primer pocket uniforming helps keep your handloads consistent, it pays to understand primer anatomy. Primers consist of a small amount of explosive mixture contained in a metal cup. Inside the cup, a small metal anvil projects into the explosive mixture, with its apex directly under the center of the primer cup, where the firing pin will strike. When the firing pin strikes, it drives the outer skin of the primer down onto the anvil, causing the explosive to detonate, which in turns shoots a spark through the flash hole that ignites the powder charge in the cartridge.
If there are differences in how high or how low primers sit in the cartridge cases, then there will be variations in how the firing pin strikes the primers. This will in turn cause slight differences in ignition of the powder, leading to variations in velocity that then cause shifts in point of impact on the target. Making all the primer pockets exactly the same depth and shape goes a long way toward eliminating any variations as early in the chain as possible - thus primer pocket uniforming is the reloading equivalent of nipping the problem in the proverbial bud.
The Tool For The Job
Sinclair’s Primer Pocket Uniformer (#749-003-709) is machined from one piece of tungsten carbide, meaning it will probably be around for your great-grandchildren to use. Available in all common primer sizes up to .50 BMG, this uniformer cuts the primer pocket to a uniform depth and also squares the bottom of the pocket in relationship to the case head. Luckily, primer pocket uniforming is usually a one-time job, needing to be done only in a case’s life, ideally before the first loading.
The Sinclair Primer Pocket Uniformer also doubles as a cleaning tool for primer pockets that have already been uniformed and fired. Just a quick touch with the tool instantly scrapes any carbon fouling or residue left after firing. If the carbon is allowed to build up in the primer pocket, it can actually cause the same problems that primer pocket uniforming was supposed to fix. Even after the primer pockets have been uniformed, it’s still important to keep them clean.
Use The Power
You can also use the Sinclair Uniformer Screwdriver Adapter (#749-001-880) to chuck the uniformer into any power screw driver, drill or the Sinclair Power Center (#749-008-418) and use it under power. This works great for both uniforming and cleaning, and it’s a real time-saver if you’re prepping a lot of cases. If you reload several calibers, and just want one simple kit to handle every possible uniforming job, the Sinclair Deluxe Primer Pocket Uniformer Kit (#749-011-518) includes everything you need in one handy hard case for storage and transport.
The primer pocket is where the firing sequence begins. It’s important that the first steps in that process are completely predictable and repeatable. Taking care to deburr the flash holes and then uniform the primer pockets are two actions that will go a long way to making your handloads and consistent and precise as they can be. Next month, we’ll look in detail at the tools and techniques for trimming cases.