I wrote a bit, but it seems to have been lost.
FIT is everything.
From Missouri Bullets:
Hardness-Optimized Bullets
Most cast bullet shooters don't know a lot about the properties of the lead alloy they're shooting because they haven't been educated about it.* If you want to learn a little bit about some important cast bullet facts, then please read on.
A common conception is that when it comes to lead bullets, harder lead equals less leading.* This is a false perception!* To explain this surprising statement, it is necessary to discuss the physics of getting the bullet out of the barrel and how lead residue comes to be deposted in the bore.* When the powder charge ignites, pressure is generated.* This pressure is measured in “copper units of pressure” (CUP) and expressed in thousand of pounds per square inch.* The heavier the powder charge, the greater the CUP.* Naturally, the purpose of generating pressure in the cartridge case is to force the bullet out of the case mouth and on down the barrel.
Lead is a soft metal.* Its hardness is expressed on a standard scale, called the Brinell Hardness Number (BHN.)* The BHN of the bullet interacts with the pressure generated by the burning powder.* The mechanism of this involves the effect of the generation of thousands of pounds per square inch of pressure which causes the base of the bullet to expand, or “obturate”.* Properly obturated, the base will have expanded beyond its original diameter which has the effect of “sealing the bore” against the explosive pressure of the gases burning behind it.* Properly sealed, and working in conjunction with the lubricant in the lube groove, the bullet will thus not allow gases to escape forward from around the base of the bullets, which prevents it from shaving lead from the bullet body and forcing it into the bore grooves (otherwise known as “leading”.)
This failure to obturate (“seal the bore against onrushing gases”) causes leading which is a chore to clean and is a major obstacle to accuracy.
An optimally hard lead bullet is simply one which obturates at a given pressure sufficiently to seal the bore against the gases which would otherwise “cut through” the soft lead (called “gas-cutting”, forcing molten lead into your rifling.* A bullet which is too hard won't obturate and seal the bore, because the gas pressure is insufficient to expand the base of the bullet.* A bullet which is too soft at a given pressure will experience excessive base expansion and vaporization of the lead, causing leading.
There is a formula for optimal bullet hardness which is simple and it is worth knowing:
Optimum BHN = CUP / (1422 x .90)
The CUP of your reloads is published in the reloading manuals.* Take a typical .45 ACP load, using a 200-grain LSWC bullet – 5.0 grains of Bullseye.* This load develops 900 FPS and is in common use among IPSC and IDPA gunners.* The reloading manual shows that the pressure generated by this load is 20,000 CUP.* So, the formula for optimal bullet hardness is
20,000 / 1279.8 = 15.62
There it is! *For this application – shooting a 200-grain LSWC at 900 FPS requires that you use a bullet with a BHN of 16 to 18 (round upwards a couple of BHN points for flexibility.)
You may be asking why shooters don't know much about this whole bullet hardness optimization business.* The reason is basically that the large manufacturers, for ease of production, use a standard alloy for all of their cast bullet construction, an alloy which has a Brinell Hardness Number of approximately 24.* Why do they do this?* It's simple – one standard alloy simplifies logistics for the big manufacturers and, equally importantly, a bullet this hard ships well by standing up to getting dinged around during transportation.* The fact that their bullets are too hard and cause leading and aren't very accurate because of improper obturation is something they'd really rather you weren't aware of.* This explains why neither their packaging nor product information will ever refer to the BHN of their products.
Along those lines, how many boxes of cast bullets – from any source – state the BHN on them?
At the Missouri Bullet Company, we optimize bullets for your intended application.* We don't take a “one size fits all” approach to manufacturing your bullets.* Every box of our bullets displays the BHN, which we constantly sample and monitor.* We take the time to create lead bullet alloy that is specialized for the bullet hardness that works best for you.* It is a fact that we spend significantly more time alloying our lead than we do in making the bullets that come from it and we do this to provide you the right bullet for your application.
From Norm Johnson:
Fitting Lead Bullets to the Gun
Shooting Lead Bullets In Handguns
Norman F. Johnson
**** A number of handgun shooters get discouraging results in their initial attempts at cast bullet reloading for some of the same reasons that new cast bullet rifle shooters experience poor performance. These poor results are usually due to a lack of understanding of what is needed to make cast bullets shoot well. In the following paragraphs I will attempt to bring into focus a few simple rules that need be followed in order to allow even the novice cast bullet loader to produce acceptable, if not outstanding, results. I will also attempt to dispel some old wives' tales that seem to abide, mostly because glossy gun writers pass on these misconceptions to shooters decade after decade.
**** The first and most important characteristic of the cast bullet is that its diameter be properly fitted to the particular gun in which it is to be fired. Right up front I would like to make a statement that knowledgeable cast bullet shooters have come to accept, quite correctly, as a universal truism - undersized bullets, even those that are undersized by only 0.001" will very often allow serious leading and its associated loss of accuracy.
**** The second statement that should be remembered is that hard bullets are not necessarily desirable for your own particular loads - sometimes softer, even much softer bullet alloy, will work better. We will expand on that statement in some of the following paragraphs.
Bullet Fitting
**** The chambers of firearms have a nomenclature that needs be to be understood. Starting with the base, we have the main body of the chamber. Ahead of the body is the neck. In a straight walled case the neck is merely a continuation of the main body and is indistinguishable from it. The bottleneck case has a neck that is reduced in diameter to hold the bullet in a firm grip. Since this is a handgun chapter, we will deal mostly with the straight case but the fitting principle is the same for both case types.
**** Immediately ahead of the case neck is the chamber throat. This throat has also been called the ball seat or bullet seat. The throat is either straight or has a very slight taper leading to the forcing cone. The forcing cone is tapered from the end of the throat to the rifling origin. It is the diameter of the throat that is all important in choosing the proper bullet diameter.
**** When the round is chambered and fired, hot, high pressure gases begin to push the bullet out of the case and into the throat. It is here that, if a good gas seal is not realized, the bullet integrity can be compromised. As the hot gases impact on the bullet base they also tend, depending upon bullet diameter, to rush alongside the bullet between it and the throat wall, scouring the bullet. This action is properly referred to as gas cutting. The gas cutting blows a small amount of molten lead and lead vapor ahead of the bullet where some condenses on and attaches to the bore wall. The bullet then runs over the deposited lead, further degrading the bullet. In severe instances, enough lead can be deposited within five rounds to completely ruin accuracy.
**** Leading that is caused by gas cutting is easily diagnosed by examining the forcing cone of the firearm.*If leading is apparent there, it is from gas cutting. If the leading occurs down the barrel or near the muzzle, it is a bullet lube problem; either not enough lube or one that is not up to the challenges of high velocity shooting. Lube failures are rare in handgun loads because of the relatively low velocities.
**** Gas cutting is eliminated, or at least significantly reduced, by making sure that the bullet diameter is no more than .0.0005" less than throat diameter. If the bullet is larger than throat diameter, as long as the resulting cartridge is not too large in diameter to chamber easily, it will work just as well regardless of the glossy gun writers claims about leading and excessive pressures. I have for many years commonly used bullets that are up to 0.007" over nominal bullet diameter in handguns where throats are grossly oversize. Unfortunately oversize throats are not an uncommon condition of either old or new firearms.
**** Remember, however, that it is not a safe practice to use oversize bullets that result in a cartridge that does not chamber easily because the neck must be allowed to expand slightly for safe bullet release. To attempt to use a cartridge so loaded is to risk generating excessive pressures.
Bullet Hardness
**** Many are under the impression that hard bullets work best. Very often, the exact opposite is true. In many cases, use of softer bullets would be to one's advantage. The reason is that the softer alloys are more easily bumped up to fill the throat (obturate) when the powder is ignited. While depending upon bump-up of soft bullets cannot ever be as dependable as correct bullet fitting, it can help in some small number of instances.
**** Forcing cone leading is nearly always the result of oversize cylinder throats or undersize bullets, not because the alloy is too soft. In fact, sometimes, for a given powder charge, a softer bullet will shoot more accurately because softer alloys will allow for more complete obturation (bumping up) and reduce or eliminate gas cutting.
**** Correct bullet hardness for revolver target loads is about 8-12 BHN, depending upon the charge giving best bullet stability and the chamber pressure generated. The usable maximum chamber pressure of an alloy is a function of its Brinnell Hardness Number. As a rule of thumb, optimum chamber pressure for adequate obturation is about four times yield strength.
**** Within the range of alloy hardness used for typical as-cast or heat treated bullets (from 5-30 BHN), yield strength is approximated by the BHN multiplied by 480. This means that a soft alloy of 8 BHN, such as factory swaged lead bullets will stand up to about 15,000 CUP (8 x 480 x 4=15,360), and an alloy of 12 BHN will stand up to about 23,000 CUP. This corresponds to the pressures generated by 4-6 grains of fast burning pistol powders such as Bullseye, 231, Red Dot, Green Dot or 452AA, which are all well suited for the .44 Special.
**** My favorite all-purpose alloy is a mixture of indoor-range backstop lead (mostly .38 wadcutter and .22 rimfire bullets) mixed with about 1 part in 20 of Linotype to provide some minimal tin to improve casting. This stuff makes a nicely filled out soft bullet of 11 BHN. By the way, this soft alloy also shoots well in moderate .30 cal. rifle loads up to about 1500 fps, and is without peer in the slow big bores, such as the .45-70.
Revolvers
**** In a revolver the throats are the areas in each cylinder chamber immediately ahead of the portion of the chamber where the brass case rests and into which the bullet projects. If the bullet is sized so that it is a gentle force fit in the throat, all else being equal, your accuracy potential will increase greatly.
**** Measure the throat diameters and slug the barrel. If you have a gun that has throats smaller than the groove diameter, (fortunately, an infrequent condition) there is not much hope for reasonable accuracy. From an accuracy standpoint, revolvers will not tolerate an undersize lead bullet rattling down the bore.
**** When you slug your barrel, note if there is a tight spot or area anywhere in the barrel. Pay particular attention to the back of the barrel where it enters the frame. A tight spot here is common and can size down your bullet. This situation can be remedied by lapping the bore.
**** Proper bullet fit in a revolver can do wonders. I can beat all my shooting buddies any day of the week; not because I am a better pistol shot, but because I fit all of my ammo to each particular gun - a decided advantage. I learned this way back when the Redhawk first came out. Through a series of very fortunate circumstances, I ended up with a matched set of the first year run. With their badly oversize .434" throats and using conventional ammo, the very best that I could get from them was 2" machine rest groups. By fitting bullets properly, they will now do 3/4" groups all day, even with full house loads. This dramatic improvement was realized in all my revolvers and I became a better shot overnight than I ever thought that I would be. It really is worth the effort.
**** Some revolver chambers have all six throats that are virtually identical, while some vary 0.0006 - 0.0007". Most hold 0.0003 - 0.0004" variation which is good enough, in my experience. Very carefully running an oversize soft slug through all six throats will give one the diameter of the smallest throat. This diameter is optimum for bullets fired in that gun.
**** You may very well find that your bullets shoot better with no sizing. My bullet sizer is virtually retired these past 15-20 years. Many bullets, as they fall from the mold, are already undersized for many modern production revolvers.
**** Tip - I fit most of my revolver bullets so that they will be a push fit into the throats and then load the cartridges so that bullets reach way out into the throats for good initial guidance; that is, with the bullet and bore axes perfectly collinear.
**** For those who load their rounds so that the bullets crimp at the crimp groove, rather than having them extend way out into the throat, oversize bullets, even those that are larger in diameter than the throats, can provide a definite advantage. There will be virtually no gas cutting, no matter what the bullet alloy.
**** Where a really long blunt bullet is loaded way out into the cylinder, the diameter determination is a little more crucial. I shoot mostly heavy (long) blunt bullets in revolvers because they allow me to load really long, to within 0.1" of the front of the cylinder face. This is important to me because, given a snug fit, the bullet holds its axis in nearly perfect co-linearity with that of the bore for an ideal launch situation. The chambers themselves are usually pretty sloppy so that conventional ammo will not lay coaxially in the chamber. Not good!
**** My Blackhawk came with .4545" throats and a .449" bore. That is a ridiculous mismatch, yet I shoot .454 - .4545" bullets in it, in both target and "boomer" 350 grain hunting loads. In addition, all of my .44 throats run at about .434" so most of my .44 bullets are made to .4335 - .434" area, again depending on the gun.
**** There is one caveat to the "big is good" situation, however. If the bullet is so large in diameter that the resulting cartridge experiences chambering interference, the situation is NOT GOOD. This is because the case neck must have a little room to expand for safe bullet release. For this reason, for the inexperienced reloader, it is prudent to keep bullet diameter pretty close to that of the revolver's smallest throat.
Autoloaders
**** For the autoloaders, most of which do not have a throat, any bullet diameter which is just large enough to prevent leading, up to that which just allows free and easy chambering of the loaded round, will work about equally well.
**** For instance, my 1911 target gun will shoot .452" bullets w/o leading. However, because I use lots of .454" bullets for my revolvers, and because the .45 ACP cartridges loaded with .454" bullets chamber freely, I do not bother to size them at all for 1911 use. All will hold the 10 ring all day long. No exaggeration! Very satisfying!
**** Fitting of the bullet to the chamber throat and proper powder selection (meaning as slow a powder as will give one desired velocities) will do wonders for leading characteristics and accuracy.
**** The first step in determining bullet diameter needed for optimum accuracy results in a self-loader such as the 1911 is to measure the chamber diameter as well as to slug the bore. If your molded or commercial bullets are about 0.001" larger than the slugged bore dimension, try those for accuracy. If it is acceptable, you are already there. If not, measure the brass thickness for the lot of brass that is being used. Choose a bullet that will make a cartridge that is about .002" less than the measured chamber diameter. In other words, choose a bullet that will make a cartridge that will drop freely into the chamber. Here, the intent is to assure that little or no gas cutting occurs before the bullet is sealed into the bore. Do not force a round into the chamber for fear of causing soaring pressures because the brass cannot expand sufficiently to release the bullet safely. I find that the largest diameter round that will fall freely into the chamber is safe.
**** For my .45 autos, this diameter is about 0.453 - 0.454".* My .45 revolvers require 0.454 - 0.456", depending on the gun. For the .45's, if one has a mold that casts .455" bullets (.45 Colt molds), he is well on his way because, although seldom necessary, they can always be sized a little.
**** For those who are serious about shooting their cast bullets accurately, having a custom mould made is well worth the effort and expense. Custom makers like LBT will make custom molds to your specified diameter(s) for only a few dollars more than those that come from the big commercial houses.
**** One further observation: When these larger than nominal diameter bullets are used and when a roll crimp is employed, sometimes there is a slight bulge produced right at the crimp. Because the resulting cartridge will not drop freely into the chamber, this can give one the impression that the bullet diameter is too great, and the unknowing reloader might reduce bullet diameter thus negating his efforts to minimize gas cutting. The solution is to use a taper crimper judiciously. The taper crimper does have the unfortunate ability to size down the bullet if it is not properly adjusted, so approach your die setting carefully.
