October 7th, 2018

Reloading Tip: Bullet Bearing Surface and Pressure

USAMU Bullet Ogive Comparision Safety Reloading
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
USAMU Bullet Ogive Comparision Safety Reloading

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.]

Permalink Bullets, Brass, Ammo, Reloading 10 Comments »
December 10th, 2016

Bullet Base-to-Ogive Sorting Tool from Sinclair Aids Accuracy

Sinclair bullet ogive sorterSinclair International offers a handy tool assembly that lets you sort bullets by base to ogive length. Yes you can do this with a comparator tool attached to calipers, but the Sinclair tool really speed up the process when you’re sorting large quantities of bullets.

The $79.99 Sinclair Bullet Sorting Stand with Dial Indicator (item 749-011-469WS) comes with a heavy black granite base that stays put on your loading bench. The included analog dial indicator has a quick-release lever allowing easy placement and removal of bullets into the comparator. This lever allow the spring-loaded indicator shaft to pop up out of the way.

In the video below, Sinclair shows how to use the Bullet Sorting device. Sinclair recommend sorting in batches with variance no greater than .005 (five-thousandths) in base-to-ogive length. We like to hold tolerances even tighter, trying to hold spreads to .003. The special base comparators used with this tool are offered for $10.99 in .22 caliber, 6mm, 6.5mm, 7mm, .30 caliber, and .338 caliber. The sorting stand can also be used with Sinclair’s handy multi-caliber hex-style comparators (items 749-002-942WS, or 748-002-833WS, $19.99).

Permalink Gear Review, Reloading 2 Comments »
September 12th, 2015

Handloading Safety Tip — Bullet Bearing Surface and Pressure

Each Wednesday, the U.S. Army Marksmanship Unit publishes a reloading “how-to” article on the USAMU Facebook page. This week’s “Handloading Hump Day” post covers an important safety issue — why you should never assume that a stated “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. Visit the USAMU Facebook page next Wednesday for the next installment.

Beginning Handloading, Part 13:
Extrapolating Beyond Your Data, or … “I Don’t Know, What I Don’t Know!”

Last week, we addressed several key facets of Service Rifle reloading. Today, 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 (below), 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.

Photo 1: Three Near-Equal-Weight 7mm Bullets with Different Shapes
USAMU Bullet Ogive Comparision Safety Reloading

Due to time constraints, the writer used an approximate, direct measurement approach to assess the bullets’ different BSLs. 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”) 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 deceiving.

Photo 2: The Perils of Measuring Bearing Surface Length with Comparators
USAMU Bullet Ogive Comparision Safety Reloading

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.]

Please join us again next week as we examine other lesser-discussed variables that affect pressure and velocity during handload development. In the meantime, stay safe, and favor center!

Permalink - Articles, Reloading 3 Comments »
January 24th, 2013

Forster Products’ New Datum Dial Ammo Measurement System

Forster Products Datum dial ammuntion measurement system

At SHOT Show we checked out a versatile and well-designed new tool from Forster Products — the new Datum Dial Ammunition Measurement System. This handy device measures a variety of important dimensions: 1) cartridge headspace from base of case to shoulder datum; 2) cartridge length from base of case to bullet ogive; 3) bullet base to bullet ogive; and 4) bullet bearing surface (requires two tools).

The complete Datum Dial Kit consists of a dial-holder mount and three precision machined dials. The mount clamps to your sliding-jaw calipers. The mount holds a black ring or one of two supplied gold rings. The black ring has 5 precision-diameter holes that let you check headspace (on shoulder datum) for nearly all modern cartridge types.

Swap in one of two available gold rings to measure to bullet ogive index points. The two gold dials (five holes each) give you 10 different bullet diameter choices, covering the vast majority of common pistol and rifle bullet sizes. This lets you measure both base of bullet to ogive and base of cartridge case to bullet ogive. We really like this dual functionality, as it allows you to quickly sort bullets, plus you can measure your loaded rounds as they come out of the seating die. I have found that accuracy is enhanced when you maintain close base-to-ogive tolerances on your loaded rounds.

Watch Video to See Datum Dial Features and Functions

Replace a Bin Full of Comparator Inserts
It is important to understand that the Datum Dial system replaces a comparator body and a complete set of caliber-specific comparator inserts. You no longer need a box full of inserts, with one for each caliber that you load. The Datum Dial Complete Kit includes: One dial-holder mount (which clamps on your calipers), one black headspace dial, two (2) gold bullet diameter dials, and a plastic storage box. The Datum Dial Kit is available right now. MSRP is $157.00, and “street price” should be $20 or so lower. Additional dials or caliper jaw mounts are sold separately.

Forster Products Datum dial ammuntion measurement system

Permalink New Product, Reloading 2 Comments »