Some people claim to see improved accuracy when they use brass with smaller flash holes. This seems to eliminate a variable which can affect ignition.
Some people claim to see improved accuracy when they use brass with smaller flash holes. This seems to eliminate a variable which can affect ignition.
But is it worth the effort of uniforming and deburring case flash holes? And does doing so affect ignition and precision? Let’s find out.
Ignition
When reloading cases it is very important to check the flash hole for size. This is particularly important if using cases with a rimmed primer. This is because the rim serves to support the primer anvil during firing, and is responsible for limiting the amount of back pressure applied to the primer, which can blow it out or even rupture the case.
Smaller flash holes can also make the 'flash plume' skinny, exposing less powder to the ignition flame and thus resulting in slower and more reliable ignition. This is why reloaders tend to prefer smaller flash holes.
Many reloaders find that enlarging the flash hole is not beneficial, in fact it can be detrimental to powder ignition. Larger flash holes can also cause the case to erode quicker because of the larger volume of escaping powder gases with more solid residue. Those who have experimented with this have found that 70 thou is about the limit for the diameter of a flash hole without causing it to erode excessively.
Precision
Many reloaders with reloading dies believe that enlarging the flash hole size will speed up and 'better' the ignition of the powder charge. This is a myth and should not be done as it can result in a larger than safe chamber pressure which could rupture the primer or blow out the breech face.
The larger diameter allows the flame to reach the powder more quickly but also increases the volume of powder gas that escapes from the case, increasing ES and reducing accuracy. The simplest way to minimize this problem is to uniform your flash holes with one of the available tools.
Uniforming is easy to do and takes no more time than reaming the case. It will deliver consistency within the test to test variations used by most load data publishers. It is important to use a fresh deprimed cartridge case for this procedure as small pieces of cleaning media can get stuck in the flash holes and prevent proper ignition and chamber pressure.
Muzzle Velocity
The velocity a bullet travels down the barrel is determined by the force of the
propellant, air resistance and the diameter of the bore. Longer barrels generally provide higher velocities than shorter ones.
As a reloader you know that the diameter and shape of the flash hole impacts how much powder is ignited and what pressure it generates. Those factors also determine accuracy.
Most shooters will agree that uniform flash holes produce more consistent ignition and better accuracy. Many will even enlarge them to make sure they are big enough for flame to enter the case and ignite the powder charge. This seems like a lot of trouble to go through just to get a little more consistency out of their loads but most find it's worth the effort. It's easier to do than re-cutting the entire primer pocket and many hobby outlets carry a variety of numbered drill sets from.028" -.056" for uniforming flash holes.
Accuracy
Many shooters are concerned about the effects of the nipple and flash hole on consistency of ignition and accuracy. They will enlarge them with special tools, but that may be counterproductive. The manufacturers have done a lot of testing to determine the best diameter for these, so changing it can cause other problems.
One issue is that if the firing pin impacts differently, it will send different sparks into the powder column and cause variations in ignition. This is why you want to keep the dimensions consistent.
Some people believe that larger flash holes will allow more of the flame to penetrate into the case faster and ignite the powder charge better. That is not necessarily the case. The size is important, but the shape of the flash hole is also. It should be uniform in size to prevent a large amount of pressure being generated inside the case. This can cause cases to swell and blow the primer out, releasing pressure into the chamber. This can lead to dangerous conditions and is why it's important that the flash hole be drilled out or uniformed using a tool such as those produced by RCBS and Lyman.