Revolver Accuracy by Alpha Precision, Inc.
As with other firearms, accuracy of the revolver is dependent upon several factors. First, the bullet must be the correct diameter with relation to the cylinder chamber throats, and the interior barrel dimensions (the groove diameter is most often referenced, but bore diameter is important as well). When cast bullets are used, the bullet should be .0005 to .0010"larger than the barrel groove diameter. Ideally, the cylinder throat will be .0005 to.0010" larger than the bullet. To illustrate, Douglas .44 handgun barrels typically have a groove diameter of .429". The cast bullet should be .4295 to .4300". The cylinder throats should then be .4300 to .4305". Jacket bullets are more tolerant and can be groove diameter + .0010. Cylinder throats .0010" larger than bullet diameter.
When cylinder throats are undersize, they can be enlarged by reaming, lapping, or honing. If they are oversize, larger diameter bullets are an answer. Also, the cylinder can be rechambered to a larger caliber and the barrel rebored to the larger caliber (the barrel can be replaced, of course), or a custom cylinder with correct dimensions made. Cast bullets should be sized for proper throat fit, not barrel fit.
In common with rifles and pistols, the barrel crown and leade (the "funnel" area just in front of the bullet seat which aligns the bullet to the bore) must be concentric with the barrel bore. Considering arms where the chamber is cut in the barrel, the leade is usually concentric. Not necessarily so with revolvers, for the leade is the barrel forcing cone, and it is seldom concentric with the bore and almost never with the chambers, unless the chambers are line-bored. The crowns of most factory guns are off a little.
Some lack of concentricity can be tolerated and still produce a level of accuracy that is acceptable and expected. We have varying levels of acceptance depending upon the gun. One would have greater expectations for a bench rifle than a scoped bolt rifle. And the scoped bolt rifle would be expected to shoot tighter groups than an open sighted lever rifle. In handguns, one would have higher expectations from a long barrel large revolver (i.e. S&W 29 or Ruger Super Blackhawk) than a 2" snub nose revolver. One could hope for 3 to 4 inch 50 yard groups with the former and perhaps the same group size at 15 yard with the latter. The spin imparted by the rifling has a gyroscopic effect the bullet will true itself in flight as long as the errors are not too great. An example is a rifle shooting groups at 200 yards almost as small as 100 yards. The bullet is said to "go to sleep" as it stabilizes.
The bench rifle barrel crown will be close to perfectly concentric. The others mentioned, good enough. Often, good enough is not, though, for the perfectionist or customers of the custom gunsmith. A revolver will almost always shoot more accurately when the barrel is recrowned and the forcing cone recut, assuming the work is done correctly and the cuts are concentric with the bore. Improperly done, the new cuts will simply follow the original with no improvement. The amount of improvement will depend upon how poor the original was, and how concentric the recuts are. A realistic expectation is 10 to 15% group size reduction using the typical S&W or Ruger.
Taylor Throating is offered in .22, .32, .357/.38, .40, .41, .44, .45, and .475 calibers. Essentially, the barrel throat is lengthened one and one half to two calibers, and enlarged to slightly over groove diameter. The throat serves as the throat in a rifle barrel, enabling the bullet to become perfectly aligned with the bore before engaging the rifling. The "choking" effect present from tightening the barrel into the frame is removed as well. The rifling leade is a very gentle 1 ½ degrees. On average, when tested before and after using a Ransom Rest, 50 yard groups have been reduced 40 to 50%. The improvement is there using both cast and jacket bullets. I have not detected a change in velocity using cast bullets. Before and after chronographing is within standard deviation of each test. Using jacket bullets, there is a slight loss, less than 50 fps in all the tests I’ve conducted. If the barrel cylinder gap is adjusted to minimum at the same time the Taylor Throating is done, there will not be a velocity loss with jacket bullets, usually a gain of 25 to 50 fps.
I am convinced Taylor Throating produces the greatest accuracy improvement value available. Line-bore chambering will produce the most accurate revolvers, but the cost is prohibitive for many. When the barrel is accurately recrowned; the forcing cone recut concentric to the bore; Taylor Throating is almost as accurate as line-bore chambering with a savings of several hundred dollars. Line-bore chambering is accomplished by chambering each chamber in exact line with the bore, as the cylinder is locked as rigidly as it will be when the revolver is shot. The lock-up is achieved using the revolver’s own components. A slow process to be sure, but it does produce the most accurate revolver.
There is often an accuracy improvement by rebarreling with a match grade premium barrel. How great the improvement, again, depends on how good or bad the original was. We use Douglas (blue only), Shilen (stainless or blue), and Pac-Nor (blue only). The quality of these three manufacturers is tops, and the choice is usually availability, finish, twist, etc. The reasons for improvement can be many. Often the bore of the factory barrel is not concentric with the barrel threads. Factory cylinders are produced by the thousands with the assumption the bore of the barrel will be in the center of the threaded hole in the frame, the chambers are (hopefully) an equal distance from the center of the cylinder, all of whom will line up with the barrel. If the bore is not in the right place the bullet will obviously become deformed when slamming into a side of the forcing cone.
Again, the gyroscopic effect of the bullet spin comes to the rescue, and the revolver will shoot better than one might expect, but not as well as it would with proper alignment. A smoother, slightly larger forcing cone will often help, especially in conjunction with Taylor Throating. The Taylor Throating aligns the bullet with the bore so it starts spinning straight instead of hitting the rifling off-center. Often, the factory barrel has a "choke" in the threaded area due to over tightening. The bullet will be squeezed at the breech end and be undersized for the rest of its travel down the barrel. As stated earlier, Taylor Throating removes the choking effect.
There is benefit locking the cylinder tight at the time of ignition. When the lock-up is tight, the variables inherent with multi-chambers trying to align with a single barrel will be reduced, leading to greater accuracy. However, it is possible to make the lock-up too tight; as the bullet leaves the cylinder and enters the barrel, it will try to align itself by moving the cylinder into alignment. If there is just enough tolerance to align, and little excess, the ideal in chamber alignment has been achieved. If there is not enough tolerance, the chamber will be locked out of alignment, the bullet will hit the side of the forcing cone, become distorted and be inaccurate.