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From Ingot to Target: A Cast Bullet Guide for Handgunners©


Index of Additional Glen E. Fryxell Shooting Articles


Chapter 6

Throat and Groove Dimensions: Cast Bullets and Revolvers Do Mix


          Inaccuracy and leading are problems normally associated with cast bullets and often are caused by the handgun itself and not the cast bullet. Cylinder mouth diameter is often overlooked as a cause of problems with firing cast bullets. The ideal cylinder mouth diameter is about one half a thousandth over the maximum groove diameter of the barrel. Sometimes it is impossible to achieve this magical combination since the cylinder mouths are occasionally larger than the groove diameter of the barrel. In this case the bullets will need to be sized the same diameter as the cylinder mouths. Extreme cases do exist where the cylinder mouth is as much as .005 larger than the barrel groove diameter. This is an extreme case, but it does happen occasionally. The only thing you can do here is to shoot exceptionally light loads and hard or gas-checked bullets, or resort to jacketed bullets. More than one revolver has been traded off because of this situation. When the cylinder mouths are too small, they can be opened up to a larger diameter by honing or lapping. Yes, reamers can be used to resize small cylinder mouths; provided you can purchase one the exact size you need. Reamers leave a good, but  imperfect finish. After reaming each hole, the finish will need to be polished afterward with a lap or very fine abrasive cloth. This little bit of polishing will remove more material and that needs to be accounted for before obtaining the reamer. Normally about one half to three quarters of a thousandth will be removed in the final polishing process to remove the tool marks left behind from the reaming operation. The finish inside the cylinder mouth, ideally, needs to be as smooth as a well polished die since the expanded bullet will be forced through it at high speed upon firing. Can opening up a cylinder mouth cause any other problems? Yes, one. When large amounts of material are removed from cylinder mouths (say about .005" or so), a burr and sharp ledge can form at the front end of the chamber (where the taper leads from the chamber into the throat), and where the bullet begins its journey into the cylinder mouth. As the bullet leaves the cartridge case and enters the throat, the burr and/or sharp ledge actually shears off a ring of lead and leaves it in the front of the chamber. This reduces the diameter of the bullet (probably asymmetrically), causing loss of accuracy, leading and deformation of the driving bands. A polishing or lapping tool will be needed to remove that sharp edge and will be turned from the rear of the chamber. Brownells sells a product called the flex hone and it might be enough of a lap to remove a sharp edge or small burr. 

          Barrel constrictions. Restrictions can occur over the threaded area of the barrel where it screws into the frame. This mechanical malady is loosely called "thread crush" in the machinists’ trade. It is more relevant in the larger calibers where the barrels are much thinner than, say the .357 variety of calibers. The thinner, more fragile, .44 and .45 caliber barrels crush more easily, and it is not uncommon for these big bore barrels to have a slight constriction just beyond the forcing cone as a result of this phenomenon. Sometimes this constriction is modest enough that it's not a problem, and in other cases it can completely ruin a sixguns accuracy. In these cases, the constriction needs to be removed before cast bullets can be shot successfully. Some success with removing this restriction has been obtained by fire lapping. Fire lapping is nothing more than impregnating cast bullets with lapping compound and shooting them out of your revolver (preferably at modest velocity). Several commercial cast bullet companies sell fire lapping kits. “Beartooth Bullets” sells cast bullets specially made for fire lapping.

          If you are a good enough gunsmith or bench fitter, you will be able to hand lap the restriction without the fire lapping. A barrel can be ruined from improper lapping practices, so hand lapping is best left to the trained and experienced hand.

          After reading the above information on how to check out and prepare your revolver for cast bullet shooting, you may well ask why it is necessary for the bullet to have such a perfect transition from the chamber for it’s journey down the bore. Well, consider what it is we are doing with the cast bullet. Basically, the cast bullet is a slug of nothing more than cheap solder that is lubricated, loaded into a cartridge case in front of an adequate charge of propellant which, when ignited becomes a mass of extremely hot gas, forcing the slug ahead of it into the tube of twisting spiral grooves. The spiral grooves cruelly force themselves into the sides of the bullet now speeding through the bore, perhaps as fast as 1600fps. Three factors come to play against the bullet. Hot gas from behind, rapid forward motion and the resistance and damage caused by the rifling. In short, we need to tune our firearm to be as kind to the bullet as possible. Restrictions (reduced diameter) anywhere inside the cylinder mouth or barrel cause the bullet to be reduced in size. After the bullet leaves the tight spot it is smaller than the remainder of the spinally grooved tube through which it has to travel. The bullet then being smaller than the bore has lost its ability to keep the hot gasses safely sealed behind it. Once this seal is broken, the hot gases are free to rush past the delicate sides of the bullet and act as a circumferential cutting torch blowing liquid alloy ahead of the bullet essentially tinning the bore ahead of the bullet causing even more lead to be wrenched from the already damaged circumference of the bullet. What all this boils down to is severe leading and poor accuracy. These are the two main reasons why many shooters are scared away from using cast bullets. Ideally, a cast bullet should  be fired through a long taper. A taper of about .0015 is ideal, if it can be achieved. The old time barrel makers who made match grade target rifles actually lapped a long taper into their barrels tapering from .002 larger at the breech to minimum diameter at the muzzle. This allowed the bullet to maintain a positive gas seal though the entire length of the bore. If such a condition could be achieved in a revolver starting with the cylinder mouth, that revolver would shoot cast bullets with perfection. Tapered barrels are not commonly encountered today. Smoothness and continuity of diameter throughout the cylinder mouth and bore will ensure accuracy and cleanliness in shooting.


Table of Contents Continue to Chapter 7 - Leading -- The Cast Bullet Nemesis  
Index of Additional Glen E. Fryxell Shooting Articles

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