A primer is just one part of the equation for reloading ammunition. Other factors include brass selection and preparation, powder selection and temperature stability.
Primers are very important for reloaders especially when shooting competitively like USPSA or IDPA and even for hunters. Primers can make or break a load.
- Composition
The primer is the little guy in the reloading process, but it's got big explosive power. Primers contain a primary explosive that's volatile enough to ignite from the firing pin strike, setting off the larger secondary explosive in the cartridge.
They need to be stored in a cool dry place. Extreme heat, cold or humidity can cause them to deteriorate and detonate. This is why you don't want to store them in garages, sheds or attics.
Also avoid handling the primers as much as possible. The oils from your hands can interfere with reliable operation later. It's best to use a small primer tray that you can dump directly from the box into so you don't need to handle them at all. This will also prevent any accidental primer spills.
- Size
There are many different primer sizes available. Choosing the right one for your particular load is important. It can impact the seating depth of your bullet and thus your accuracy. For plinking loads, this probably isn't as critical but for benchrest or PRS style competitions it can be.
For example, a small rifle primer may work just fine with jacketed bullets but if the cartridge is loaded for lead it calls for a magnum primer. A magnum primer has a larger flash hole to better ignite a large powder charge.
Regardless of what kind of primer you choose it is important to follow published loading recipes exactly. Switching primers can cause inconsistent velocities or spike pressure to unsafe levels. It is not uncommon for a swap to a different primer brand to create velocity variances of 100 feet per second or more and pressure variances in the thousands of pounds per square inch.
- Ignition
The primer’s ignition ignites the gunpowder inside a cartridge, propelling the bullet forward. The reloader’s job is to ensure the correct size and type of primer matches the cartridge to maximize performance.
Choosing the right primer is a complex task, but it’s essential for getting the most out of your ammunition. Experimentation under safe conditions is the best way to evaluate a primer’s performance and find its ideal match for your handloads.
Other factors to consider include brand, temperature sensitivity and magnum vs. standard primers. Magnum primers can be a huge advantage in some applications, but it’s important to make sure the primer is matched to the cartridge and shooting conditions. A primer that’s not the right match can lead to a misfire or inconsistent firing.
- Temperature
The temperature of the primer is important as it controls the annealing and extension steps of PCR. For optimum results the annealing temperature should be about 5degC below the Tm of the primer pair.
It is also important that the primers have an optimal melting temperature, which controls how well they bind to the template DNA during PCR. The melting temperature depends on the composition and length of the primer, as well as its GC content.
The number of G or C bases in a primer should be limited because too many can cause cross dimer formation. In addition, long runs of the same base can misprime. For this reason it is best to avoid having too many consecutive di-nucleotide repeats in the primer sequence, ideally 2 or 3 repeated bases.
- Accuracy
Choosing the right primer is just one piece of the puzzle when it comes to creating consistent ammunition. But a quality primer is essential to getting the most out of your powder charge and your rifle or pistol.
Match primers, which have a harder cup to help them stand up to higher levels of pressure, can offer more reliable ignition and are often preferred by benchrest shooters and for varmint hunting applications. However, a standard primer can still deliver very good accuracy in these applications as well.
In the end, it is a matter of personal preference and what type of shooting you do. Always start a new load work-up when substituting a different primer into your recipe. Otherwise, you could be wasting your time and money on an unreliable primer.