One of the more recent discussions to be circulating within the reloading community is that of bullet depth. Aided by new products that have come onto the market such as the Hornady OAL Gauge the re-loader has never been better equipped to determine the optimum bullet seating depth. So what is bullet seating depth and what does it matter. Most commercial ammunition is manufactured to make sure it fits the vast majority if not all rifles that it may be used in. In magazine fed rifles the ammunition also needs to fit into the magazine itself. As a result the depth at which the bullet is seated into the case itself is not necessarily optimized for accuracy but instead optimized for conformity. By ensuring that the ammunition will fit all rifles it is fairly certain that a gap will be left between the ogive of the bullet head and the rifling of the barrel. This gap is called the “jump” and can fundamentally affect the accuracy of the rifle/ammunition combination.
The re-loader and target shooter aren’t normally restricted by the confines of magazines or having to ensure their ammunition works across all rifles. They have the option to hand feed their rounds into the chamber and most likely only one rifle to worry about so the option to seat the bullet at the optimum depth rather than a compromise depth exists enabling the optimum accuracy to be obtained.
The easiest way to determine the OAL (Overall Length) of the cartridge (sometimes referred to as COAL or C.O.L – cartridge overall length) as best suits your rifle is to use one of the tools on the market that allows for this. The OAL Gauge from Hornady is the one that I have used and consists of a metal casing with a thread at the end that is hollow and allows for a plastic rod to slide through it. To one end a slightly oversized empty cartridge case is screwed and within that a bullet head can be placed. As a result by moving the plastic rod the bullet head slides up and down within the case. The entire unit is then placed into the breach of the rifle and the rod moved until the bullet head engages with the lands and grooves of the rifling. It is recommended that a cleaning rod be used down the barrel to push lightly against the bullet head so it is not jammed into the rifling. Once the perfect balance between the two rods pushing against either end of the bullet head is reached the plastic rod can be locked in place and the unit withdrawn from the breach. The OAL of the cartridge can then be measured with calipers either to the tip of the bullet itself or ideally to the ogive of the bullet using a collimator. This latter approach offers greater consistency as the tip of the bullet will vary across bullet heads.
Once the OAL has been measured the shooter can then determine the best overall length for their cartridges. Usually the re-loader will take two to three hundredths of an inch off the OAL to leave a small jump. For example on a recent reloading of 338 Lapua Magnum my measured OAL was 3.98” so for one batch of reloads it was determined to set the OAL at 3.95”. Pictures of the difference between standard UK Army issue 338 Lapua Magnum using RUAG brass and Lapua Lockbase bullet heads and home loads using RUAG brass and Berger VLD 300 grain bullet heads can be seen on the Figure 14 Facebook page at http://www.facebook.com/figure14ltd. These are not recommendations of course and the reloader must determine his or her own optimum depth.
Many reloaders will create several batches at slightly differing OALs and even with slightly different powder loads as well and then shoot those combinations to determine the absolute optimum settings for their rifle.
As with all reloading the appropriate level of experience, care and precision should be taken. Always operate within the manufacturers guidelines for starting and maximum loads and always check your brass. Check out this and further blogs at the Figure 14 website at http://www.figure14.com
Figure 14 is a partner of THOR Global Defense Group and Knesek Guns importing THOR and EDM rifles into the UK