If you sell rifle ammunition, then Hexagon Ammunition’s ballistician Paul Bradley is here to help you understand the complex world of seating depth which is a hot topic among homeloaders. Here, he explains some of the jargon and debates to help you sell more in store
Shooters who load their own ammunition are not bound by much regulation. The onus is on them to ensure it is safe for use in their personal firearm. Of course, they also want to make it as accurate and precise as possible. One of the many factors influencing velocity potential and precision is the seating depth of the projectile, which also defines overall length to a large degree. Seating a projectile further out leaves more space in the case for propellant (higher velocity potential) and decreases the distance between the projectile and the lands (beginning of the rifling). Unless the projectile is butted right up to the lands, it will need to jump through the throat of the barrel before meeting significant resistance (squeezing into the rifling). The amount of jump can influence precision.
There are many theories as to why this may occur, but the most significant is pressure consistency. If the bullet engages with the rifling, giving consistent pressure within the chamber each time, it will have a positive effect on precision. The amount of jump which gives the best results varies by platform. It may surprise some to learn that I found longer jumps than the magical 20 thou (0.020”) to give better pressure consistency. I commonly load to 0.070” of jump and fine-tune from there.
When talking about jump, we must also bear in mind that the internal dimensions of our barrels change over time due to erosion. Hot gases flow across the metal and erode the bore more significantly than some may imagine. Testing done on calibres which are popular in PRS competition showed an average of 0.004-0.007” per 100 rounds. To truly tune our jump would be a constant process. This is a good reason to use a longer jump as it provides a bit of a plateau in terms of pressure variability.
The other factor we must take into account is safety. We know that seating depth can influence pressure. On the extreme side of things, a bullet jammed into the lands can raise chamber pressure significantly—enough for a catastrophic failure. You can use an OAL gauge to measure the seating depth to the lands using your chosen projectile. Once you have established this in your system, it is simply a case of shortening the seating depth by 0.070” and fine-tuning from there. If you start at the often-mentioned 0.020”, you have far less room for error.
It would also be wise to mention how one might measure the overall length of a round. The obvious way would be with a Vernier caliper from bullet tip to case head. The problem with this is that bullet tips can vary slightly (especially HPBT). This can give one the illusion that the distance between bullet ogive and lands has changed when, in fact, only the tip has. It is far more consistent to use a bullet comparator, a tool which slips over the end of the bullet giving a consistent flat base to measure from.
Now we get onto the subject of factory ammunition. Unlike home loads, ammunition sold by the factories must adhere to specific regulations. This may be CIP, SAAMI or NATO spec, depending on the region and end user. The maximum overall length of a cartridge is specified and strictly adhered to, therefore chasing the lands is not a consideration. The requirement with factory ammunition is not the ultimate performance in one single gun; it is the ability to work well in all relevant firearms. Thus, the specifications exist to ensure safety and function. They must take into consideration the difference in freebore from one rifle to another. The throat (freebore) can vary widely, and I have seen rifles which have over an inch of it. That is some jump. Even in that rifle, we were able to achieve sub-minute performance by selecting the correct propellant, charge weight, and primer. This goes to show that while seating depth plays a part, it is not the largest variable by any means when it comes to tuning for precision.
One might imagine you would just build all ammunition to max CIP/SAAMI length. The max length specified does not mean it will fit in a firearm’s magazine. A good example would be .338 Lapua Magnum. Some older magazines (and still popular) will not accommodate max CIP length. A 250gn SMK seated at 93.5mm (max CIP length) will not chamber in some rifles due to a very pronounced ogive form factor. I saw a customer learn this to their detriment after insisting on max overall CIP length and then being unable to close the bolt on their SAAMI spec rifle. CIP and SAAMI are interchangeable though, right? The answer is, sometimes!