When it comes to cast, is harder always better, a higher Brinell #?, and if not, what would be a softer cast bullet be useful for?
Thanks
Ken
http://www.lasc.us/castbulletnotes.htm
Short answer: Harder is usually worse, at least in todays world of 20+ BHN bullets.
The primary cause of leading and poor accuracy is bullets that don't fit the bore. The result is blow by around the bullet from the hot gas (plasma) generated by burning powder. This hot plasma melts the side surface of the bullet where it passes, and that melted lead gets left behind degrading the bore condition.
There are two factors that ensure proper fit of the bullet to the bore.
First, you have to slug that bore and find out how big it's grooves actually are. When you do you will learn something. As you drive a soft lead slug or fishing weight through the barrel you will find the resistance varies. That is because not gun bore is perfectly the same diameter and round shape all the way through. But, what comes out the far end will be an approximation of the smallest the barrel reaches. For a 10mm this should ideally be .399" to .400". But several of my guns run .401"+ (Glock). You bullets need to be at least .001" larger than the measured groove diameter of your barrel. This is why most commercial cast 10mm bullets are .401" - The barrel is supposed to be .400". If yours is larger than .400", you need bigger bullets, or you should expect poor (and sometimes dangerous) results.
Second we have to account for those inconsistencies in the bore diameter, or at certain points the plasma will still get by the sides of the bullet. This one of the reasons you will see leading only at certain places in the barrel. This is accomplished by matching the bullet hardness to the pressure of the load you wish to shoot.
Deformation of metal can be described as elastic or plastic. Elastic deformation occurs when the pressure is high enough to alter the shape of the metal, but not permanently. Plastic deformation occurs when the pressure gets high enough that the material does not return to its original shape once the pressure is removed. Plastic deformation of the bullet base is bad for accuracy. It has the same effect as a poor crown, eg the bore seal does not break consistently around the base of the bullet which results in a directional "push" off axis of travel, and a spin of larger concentricity.
By ensuring the pressure of our load reaches into the elastic deformation range of the lead alloy we are shooting, but does not reach the plastic deformation range, the bullet base swell to seal, or obturate, the bore.
Target shooters of the past favored pure lead wadcutters for their bullets. These very soft lead bullets with a BHN number of 5-6 have a broad elastic range. This makes them forgiving in one sense. But owing to their softness they also have a very low threshold for plastic deformation. They also lack toughness making them more likely to fail to hold the rifling. The physics of straight wall cases and typical section density ratios mean these bullets are usually limited to velocities below 800 fps. But, keep in mind that it is pressure that creates this limit, not velocity. With a longer barrel, significantly higher velocities are fine, as the pressure still falls within the elastic profile of the bullet material.
Tin was commonly used to harden bullets for greater pressure, and correspondingly more velocity. Tin content is typically measured in a ratio such as 30:1, 20:1, 14:1 or 10:1, lead: tin. The lower the amount of lead, or higher the tin percentage, the harder the bullet will be. Tin is suited for bullets used for taking game as the alloy is still tough and malleable. Other alloying elements, like antimony, can result in larger gains in hardness but are less ductile and more brittle.
Due to lack of understanding of these principles shooters that have issues with leading demand harder bullets than they really need. The result is a difficulty in finding commercially cast bullets today with a BHN under 20 in most cases. Such hard bullets are attained economically by using lead/antimony/tin alloys that are quenched in water from the mold. The bullets are very hard, and usually settle in around 22-23 BHN after aging, though straight from the quench they can be as hard as BHN 30. However, they are also very brittle comparatively.
I find a BHN of 14-16 to be an ideal all around hardness. I further find that 96/3/1 lead/antimony/tin bullets cast from 700° lead, air cooled and aged for 4-6 months will measure nicely in this hardness range. I shoot this alloy in my everything from 9mm to 300 win mag, with appropriate loads, using gas checks for most rifle shooting. It is relevant to note that this BHN sweet spot is not my discovery. Elmer Keith favored 14:1 lead:tin which also was about 14-16 BHN, and is a bit better suited to hunting with it's increased malleability.
I have just scratched the surface of cast bullet theory here. The LASC link above will provide hours of good reading, as well as some apparent contradictions that can be unraveled if you think on them long enough.
I believe the cast that I am currently loading is either 15, or 17, so I should be good there. Thanks by the way, this info is very informative
Ken