A rec.guns poster wrote:
The other question involves the throat diameters of a cylinder. One book I have states that the correct throat diameter is the same as the groove size or a bit larger. But on my new 357Mag the throats measure .359 to .360. The question is are the throats too large and will this affect accuracy.
Knowledgeable cast bullet shooters look at the cylinder throat dimensions as they are related to bore dimensions very carefully when they purchase a gun. 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. Two factors come into play here to improve potential accuracy: a) the bullet axis is forced to be more nearly coaxial with the barrel axis than it would be if it were allowed to lay cocked at an angle due to tolerances between cartridge and cylinder chamber dimensions. b) gas cutting, which will positively RUIN accuracy, is virtually eliminated. The above is true of cast (non-jacketed) bullets more so than jacketed because cast bullets are more subject to gas cutting but the importance of concentricity of the two axes should not be under estimated. Using an old C-H Swage-O-Matic bullet swaging press and appropriately modified (opened) dies I am able to reshape and resize most pistol bullets, including most of my cast bullet rejects, to usable and usually very accurate projectiles. If one does not require that the increase in diameter exceed a couple of thousands, and the bullet is not too hard, (no jacketed bullets here) a bullet sizer with the appropriate die can sometimes be use to "bump up" a given cast or swaged bullet to proper size although this does not provide the same control and consistency as bullet swaging equipment. Be careful though, it's easy to break the sizer handle. I swage bullets (both cast and jacketed) to fit my .44 Bulldog cylinder throats snugly. My Bulldog's throat is oversize (.4340") as are most of the revolvers of all makes that I have owned in the past few years. > How would I check/alter this?> > You can check this by pushing a bullet through the throats of your revolvers. If they drop through freely, the bullets are undersized. IDEALLY, the throat should measure one or two thousands larger than your MEASURED bore and the bullet should be about half a thousand less than the throats (or the size of the smallest throat). Throat diameters will vary about half a thousand on a typical cylinder. I can say from experience that if your guns are of recent manufacture they will have oversize throats. The only way to correct this problem (from a practical non- machinist standpoint) is to send the gun back. I have done this with three revolvers in recent years and new guns were sent to me with ideal dimensional relationships. It seems that the companies will respond if one can relay the message that he understands the problem. There is a work-around for over sized throats that does not require gunsmithing. Hollow based wadcutters were designed so that the impact of the powder gases on the hollow of the bullet would obturate the skirt to fill the throat and bore - thus eliminating gas cutting. This solves part of the problem, which, for lead bullets is the most crucial part. Should the throats be undersized in relation to your bore (a condition that is rarely encountered) they can be opened up by a machinist or done at home using number 400 wet-or-dry sand paper on a split dowel. This must be done very carefully, however, making sure that the dowel is not tipped in the throat or an awful out-of-round mess will result. Also, be aware that if the area of the bore where the barrel screws into the frame is necked down, all the bullet fitting in the world will not help. Unfortunately, this is a pretty common condition of guns coming off the line these days. Veral Smith's fire lapping kit can help to correct this condition as well as smoothing up the bore. Do not use valve grinding compound as it is too coarse. He can be reached at Lead Bullets Technology. Lead Bullets Technology HBR 62, Box 145 Moyie Springs, ID 83845 (208)267-3588