Reloading Tip: Bullet Bearing Surface and Pressure
Photo 1: Three Near-Equal-Weight 7mm Bullets with Different Shapes
TECH TIP: Bullets of the same weight (and caliber) can generate very different pressure levels due to variances in Bearing Surface Length (BSL).
Bullet 1 (L-R), the RN/FB, has a very slight taper and only reaches its full diameter (0.284″) 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!
This article from the USAMU covers an important safety issue — why you should never assume that a “book” load for a particular bullet will be safe with an equal-weight bullet of different shape/design. The shape and bearing surface of the bullet will affect the pressure generated inside the barrel. This is part of the USAMU’s Handloading Hump Day series, published on the USAMU Facebook page.
Beginning Handloading, Part 13:
Extrapolating Beyond Your Data, or … “I Don’t Know, What I Don’t Know!”
We continue our Handloading Safety theme, focusing on not inadvertently exceeding the boundaries of known, safe data. Bullet manufacturers’ 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.
Bullet bearing surface length (BSL) is often overlooked when considering maximum safe powder charges and pressures. In photo 1 (at top), note the differences in the bullets’ appearance. All three are 7mm, 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.
Bearing Surface Measurement Considerations
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 deceiving.
Photo 2: The Perils of Measuring Bearing Surface Length with Comparators
In Photo 2, two 7mm comparators have been installed on a dial caliper in an attempt to measure BSL. Using this approach, the BSLs differed sharply from the original [measurements]. The comparator-measured Bullet 1 BSL was 0.694” vs. 0.324” (original), Bullet 2 was 0.601” (comparator) vs. 0.430” (original), and Bullet 3 (shown in Photo 2) was 0.602” (comparator) vs. 0.463” (original). [Editor’s comment — Note the very large difference for Bullet 1, masking the fact that the true full diameter on this bullet starts very far back.]
Similar Posts:
- TECH TIP: Bullet Bearing Surface Length Can Affect Pressure
- How Bullet Bearing Surface Length Affects Case Pressure
- How Bullet Bearing Surface Length Can Affect Load Pressure
- How Bullet Bearing Surface Length Can Affect Load Pressure
- Handloading Safety Tip — Bullet Bearing Surface and Pressure
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I’ve done this a lot throughout the years. I think the most weight difference was 3 grains. I’ve never had a problem nor have I encountered pressure issues on the starting charge weight using different style Bullets than listed. I don’t mix monolithic bullet data with a cup and core bullet.
So you told us what not to do but provided no guidance on how to do it right. Is there a Part 2 to this?
Also, the bearing length will be different on the lands and grooves.
I’ve measured the hole diameter on inexpensive comparators. The diameter seems to be the nearest letter drill size, not very close to the bore diameter.
The reason for measuring bullets is to sort for close-to-same-value for the same bullet SKU (ID/part number) – whether you are sorting for length, weight, or any other value. In actual reloading, why in the world would you sort different bullet SKUs? Are you going to shoot different bullet SKUs at the same target? Using two comparators works fine because you sre comparing the same bullet SKUs and the same value – here BSL. That noted, since the bullets seem to rarely seat into the comparators exactly the same each time, I take the caliper value that comes up the second and third time (digital caliper value).
If you don’t like this approach you might try base-to-ogive – a comparator at one end and the flat of the bullet base against the flat edge of the caliper. This seems to work fine on lathe-turned bullets that are finished off (parted off) very cleanly. Jacketed bullets not so well because the shape at the base seems to be less consistently “flat”.
All I got to say is: PROVE IT
Nobody EVER has
I found that using the base-to-ogive comparator method to be unreliable. I have sorted bullets using this method , then measured them again and then again. Each reading was different, sometimes quite significantly. Even rotating the bullet and remeasuring did not solve the problem. Using two of these comparators doesn’t solve the problem because it’s so difficult to get them perfectively aligned. Then, I found Shooting Shed (UK) who makes a self aligning two piece comparator for measuring BSL. That solved the problem nicely and my results show it.
Lynn – I assume you are talking about the SHED 5-P, described here – http://shootingshed.co.uk/wp/2016/08/they-steal-things/ (third paragraph from the bottom). Since the contact area is very slightly angled (the source of the alignment problem), how does this tool align the bullet without it being built for that exact bullet angle? Actually, seems (second paragraph from the bottom) like that is exactly what he does. What data did you need to provide to get your 5-P?
The above discussion is why I do multiple readings and only take the value that comes up the same the second and third times. Ah the beauty of good (Mitutoyo) digital calipers.
The set I’m using is the SHED 2-P Series Bearing Surface Comparator at:
https://shootingshed.co.uk/oscom/product_info.php?cPath=38&products_id=171
It is caliber specific and very repeatable.The results I get with my reloading process, of which this is only one small part, are groupings better than 0.1 MOA, this measured on a 200 yard range (the longest I have access to). I might also point out that this is benchrest and a very, very good rifle.
What is the end effect of my measuring BSL this way? I don’t know, but I recall it having improved from using a single Redding BSL tool.
The article makes an interesting statement about bullet geometry and that measured pressures from bullets with different bearing surfaces did not match, but I am curious about the experimental procedure. Was an attempt made to differentiate between pressure difference caused by the differences in bearing surface lengths and the fact those longer bullets would have to be seated either deeper into the case or closer to the throat, both of which are known to raise pressure when taken in isolation?