WARNING: All technical data in this publication, especially for handloading, reflect the limited experience of individuals using specific tools, products, equipment and components under specific conditions and circumstances not necessarily reported in the article and over which the National Rifle Association (NRA) has no control. The data has not otherwise been tested or verified by the NRA. The NRA, its agents, officers and employees accept no responsibility for the results obtained by persons using such data and disclaim all liability for any consequential injuries or damages.
Previously, we addressed several key tenets of Service Rifle reloading. Today, we continue our handloading safety theme, focusing on not inadvertently exceeding the boundaries of known, safe data.
Bullet manufacturer’s loading manuals often display three, four or more similar-weight bullets grouped together with one set of load recipes. The manufacturer has tested these bullets and developed safe data for that group. However, seeing data in this format can tempt loaders—especially new ones—to think that ALL bullets of a given weight and caliber can interchangeably use the same load data. Actually, not so much.
The researchers ensure their data is safe with the bullet yielding the highest pressure. Thus, all others in that group should produce equal or less pressure, and they are safe using this data.
However, bullet designs include many variables such as different bearing surface lengths, hardness, and even slight variations in diameter. These can occasionally range up to 0.001” by design. Thus, choosing untested bullets of the same weight and caliber, and using them with data not developed for them can yield excess pressures.
This is only one of the countless reasons not to begin at or very near the highest pressure loads during load development. Always begin at the starting load and look for pressure signs as one increases powder charges.
Photo 1: The bullets used for the bearing surface length example.
Bullet bearing surface length (BSL) is often overlooked when considering maximum safe powder charges and pressures. In Photo 1, note the differences in the bullets’ appearance. All three are 7 mm, and their maximum weight difference is just five grains. Yet, the traditional round nose, flat base design on the left appears to have much more BSL than the sleeker match bullets. All things being equal, based on appearance, the RN/FB bullet seems likely to reach maximum pressure with significantly less powder than the other two designs.
Due to time constraints, the writer used an approximate, direct measurement approach to assess the bullets’ different BSL’s. While fairly repeatable, the results were far from ballistics engineer-grade. Still, they are adequate for this example.
Bullet 1 (L-R), the RN/FB, has a very slight taper and only reaches its full diameter (0.284 inch) very near the cannelure. This taper is often seen on similar bullets; it helps reduce pressures with good accuracy. The calculated BSL of Bullet 1 was ~0.324’’. The BSL of Bullet 2, in the center, was ~0.430”, and Bullet 3’s was ~ 0.463”. Obviously, bullets can be visually deceiving as to BSL!
Some might be tempted to use a bullet ogive comparator (or two) to measure bullets’ true BSL for comparison’s sake. Unfortunately, comparators don’t typically measure maximum bullet diameter and this approach can be deluding.
Photo 2: An amateur attempt at measuring bearing surface lengths—not recommended!
In Photo 2, two 7 mm comparators have been installed on a dial caliper in an attempt to measure BSL. Using this approach, the BSL’s differed sharply from the originals. Bullet 1’s new BSL was 0.694” vs. 0.324”, Bullet 2: 0.601” vs. 0.430” and Bullet 3: 0.602” vs. 0.463”.
Please join us next time, as we examine other lesser-discussed variables that affect pressure and velocity during handload development. In the meantime, stay safe, and favor center!