Practical Effects of Bullets on Pressure
January 04, 2011
Remember the old motor oil commercial on television where the old guy kept saying, "Oil is oil," as his car belched forth a huge cloud of white smoke?
The point of the commercial, from the standpoint of the company that coughed up the big bucks for it, was that oils were different, and you had to think about your choices.
Whether that was true or not for motor lubricant back then was debatable, but today's handloader must not think, "Bullets are bullets." Technology and progress have taken that option from us.
I started reloading in the late 1960s when bullet manufacturing was not much different among the bullet makers. Whether from big factories hammering out payloads for commercial ammo or from the smaller firms like Speer, Sierra, and Hornady who served the hobby reloaders, there was not much difference among component bullets. Everyone used very similar jacket materials and thicknesses, and if you lined up several brands of .30-caliber, 180-grain bullets, they looked very much alike. The only common bullet that strayed from the "same-as" syndrome was the Nosler Partition; its solid jacket partition made it longer than bullets of the same weight but of conventional construction.
Reloading data even reflected this situation. Propellant companies seldom identified the bullet they used for developing loads and, for the large part, didn't need to. Yes, changing from one bullet brand to another could change pressure, but the effects were generally small enough to remain within the safe zone. That's assuming the handloader observed the most important safety rule of load development: Always begin at the published starting load and work up toward the maximum in reasonable increments, testing each increment before shooting the next higher one.
Today, we have a broad choice of bullets to meet any shooting need. With this proliferation, the ways bullets are made have dramatically changed, creating significant differences among them that affect how much powder you load.
Let's consider some bullet characteristics that can influence load safety. For the sake of discussion, I'm talking about bullets of the same diameter and weight but of different construction.
The big hitters in bullet length are diameter and weight. A .30-caliber, 150-grain bullet is longer than a .35-caliber bullet of the same weight. Weight itself is a function of the material densities. Lead is denser--weighs more per volume unit--than copper, steel, or plastic. Pure lead is heavier than lead alloys.
Thirty years ago, the copper jackets on most big-game bullets made up 15 to 20 percent of the total weight, with the remainder being dense lead. Today the jacket is heavier, making up more than 50 percent of the total weight in premium bullets. In projectiles like the original Barnes X-Bullet, the lead is totally eliminated.
Increasing the amount of lighter jacket material relative to lead makes the bullet longer if diameter and weight remain constant. A longer bullet intrudes deeper into the case, often necessitating a charge reduction, and increases the next characteristic.
A basic projectile has a body and a nose. The body typically makes up the bearing surface, which is that part of the bullet that physically touches the barrel. It is a function of shape. Of otherwise identical bullets, the true roundnose--a cylinder with a hemisphere on one end--has the greatest bearing surface. In most cases, increasing bearing surface will increase pressure. This is where the greatest frictional forces occur.
Differences in bearing surface among different bullet brands, even within one brand, are a common explanation of why load data between bullet weights won't "track." Without lab access, the average consumer will have a hard time telling if bearing surface changes make a certain charge weight for one bullet unsafe for another.
We had a prime example of this at Speer. Three 6mm bullets weighed 100 to 105 grains. The 100-grainers were the spitzer boattail (BT) and the Grand Slam. The 105-grain was a flatbase Hot-Cor spitzer. Until Speer Reloading Manual #14, we showed separate, often lighter load data for the 100-grain BT in .243 Winchester. The original BT had a relatively short six-caliber radius nose profile, and the boattail section wasn't particularly long. Its bearing surface was longer than the Grand Slam and the Hot-Cor, both having longer nose sections. Between manuals #13 and #14, we changed the boattail's nose to an eight-caliber profile similar to the 105-grain bullet. The longer nose meant a shorter bearing surface, and retesting showed the BT could now share the same data as the other two products.
As the previous example shows, changing the nose shape or profile can change bearing surface and overall length. Any tapered section--nose or tail--will reduce bearing surface but will also make the overall length of the bullet greater.
Grooves in the bearing surface, like on the Barnes Triple-Shock, reduce the total bearing area and therefore the force required to overcome friction. This was an excellent addition to the original design whose all-copper construction resulted in a fairly large bearing area.
Most bullet jackets are a copper alloy called gilding metal, which is 95 percent copper and 5 percent tin (expressed 95/5). Some use a harder 90/10 alloy. Both are harder than pure copper and usually have lower frictional forces unless the pure copper jacket gets special processing.
Bullets with a higher percentage of light metals are longer, but where the light metal ends up can affect pressures, too. The big hitter here is jacket thickness. Varmint and match bullets usually wear relatively thin and untapered jacket walls. Big-game bullets will have thicker jackets to control expansion, and the jacket will be strongly tapered or profiled to put extra material in the critical shank area to increase retained weight after expansion.
Thinner jackets usually create higher pressure than thick ones. All bullets suffer the same dynamic forces when slammed by the propellant's pressure wave. The back of the bullet starts to move before the front, compressing the bullet parallel to its long axis. Were the bullet not confined inside a gun barrel, the jacket walls would bulge outward. Confinement does a funny thing; with pressure behind and frictional resistance ahead, the only way the bullet can deform is forward. The bearing surface lengthens at the expense of the nose length, and the bullet becomes more blunt. Elmer Keith called this deformation "slugging up" and liked it, as making any blunt bullet fit his idea of a proper big-game bullet. Additional increase of the bearing surface happens as the rifling engraves the bullet and displaces m
A stiffer jacket wall better resists this "slugging up" and can lower pressures. In standard .30-caliber Hot-Cor spitzers, the 165- and 180-grain versions use a heavier jacket than the 150-grainer, although the nose profile remains constant. If you plot the maximum charge weights across these three exact increments of bullet weight--150, 165, and 180--you see that the max load for the 165 is skewed toward the max charges for the 150. Were all three made with the same jacket, the max charge weights for the 165 would fall closer to the midpoint.
The Right Test Bullet
With several bullet shapes and variations in construction, the data developer has a challenge: How do you show safe data for all your bullets without making a manual that looks like a Manhattan phone book?
For example, Speer has several bullet shapes and styles for one weight. In .30-caliber, 150-grain bullets, there are eight separate products. Testing each showed that the roundnose Hot-Cor gave higher pressure by a small margin. That became our test bullet in most cartridges.
Did we leave anything on the table by this method? I don't think so. The differences were small, typically under 2,000 psi, and were not enough to make much difference in the real world. Use the published data to stay safe. You don't know which bullet produces the lowest pressure, and trying to "work" the system to get a few more feet per second invites danger.
What does this mean to you?
If you follow the two big rules of reloading--(1) always use published, lab-test load data and (2) always begin at the published starting load and work up toward the maximum in reasonable increments, testing each increment before shooting the next higher one--you should not get into trouble with the new high-tech bullets. But I add a third one that's more a variant of number one and was seldom required 30 years ago: Always use data supplied by the maker of the bullet you load.
I once handled a tech support call where the caller started out by saying, "Your 7mm Rem. Mag. load data is no good!" Well, he didn't exactly say, "no good," but this is a family publication. He said he was loading 175-grain bullets and some common propellant, and that he had to beat the bolt open after each shot.
I asked his charge; he had selected the maximum load in the Speer manual for the Speer 175-grain Grand Slam. He did not use the lower loads, breaking Rule #2. After a few more questions, I found out he was loading the 175-grain Barnes X-Bullet. I had some Grand Slams in my office, and I asked him to fetch some X-Bullets from his shop and measure the length. When he returned, we discovered the bullets he loaded were a full quarter-inch longer than the ones we used for data development. In interior ballistics, that's huge.
I politely advised him to talk to the nice folks at Barnes and get their loading recommendation; better yet, buy their manual.
The technical evolution of bullets has produced a proliferation of differences. The generic bullet, and therefore generic load data, is as outdated as crank starters on automobiles. The prudent handloader will maintain a representative selection of loading manuals covering the bullet brands he loads.