BULLET LENGTH AND RIFLING CHARACTERISTICS
Beyond the question of bullet diameter there is the matter of bullet length and the relationship of bullet length to the rifling twist in the barrel and how this affects accuracy. Bullets aren’t identified by length, but by weight when they are sold. All other things being equal, heavier bullets of a given diameter are longer.
One way to find out which bullets will work best in your gun is trial and error. Another way is to limit yourself to the recommendations in loading manuals. These are basically guidelines and performance for the caliber of your gun and may or may not be satisfactory to you. Beyond this there are some basic calculations which may save you a lot of time and expense on bullets that don’t work.
Therefore, a second thing you should know about the barrel of the gun you will be loading for, beyond its groove diameter, is the rate of the rifling twist. This can be found in loading manuals for a great many standard guns, certainly for the test guns used to prepare the data. This figure will be expressed, for example, as “Twist 1-10″.” This indicates that the rifling spiral makes one complete turn in ten inches. Different lengths of bullets require different rifling twists to shoot to their best advantage. If the match between bullet length and rifling is too far off, bullets may fail to stabilize and tumble in flight on the one hand or be so over stabilized they will actually break apart in flight on the other.
If there is any doubt in your mind about the twist rate of your gun, determining this is simplicity itself, at least with a rifle length barrel. With handguns, you will have to interpolate as best you can. Stand the rifle against a plain vertical surface such as a wall or door. Place a good tight patch on your cleaning rod — one that does not have a ball bearing in the handle. Once the patch is started, mark the handle and beside it make a mark on the vertical surface. Push the rod down the barrel, allowing the handle to turn freely. Make a second mark at the point where the handle has made one complete rotation. Measure the distance between the top and bottom marks and you know the twist rate to a very close degree, although there will always be a slight amount of slippage.
As a rule of thumb, longer bullets of a given caliber require a faster twist to stabilize them to the point where they shoot accurately than shorter bullets. This is true without regard to weight or velocity. The familiar 22 Long Rifle shoots best in a 1-16″ twist barrel. This holds true for 40 grain target loadings as well as 30 grain hyper-velocity hollow point hunting bullets.
These are always made to be close to the 40 grain LR bullet’s length. The stubby, 30 grain 22 Short does best in a 1-20″ twist barrel. It will stabilize in a 1-16″ barrel, but accuracy is not good. Rifles marked “22 Short, Long, or Long Rifle” are actually bored for the Long Rifle or occasionally with a compromise twist of 1-v17″, which may slightly improve the accuracy of the Short, without adversely affecting the accuracy of the Long Rifle.
Once you know the twist of your gun you can calculate which bullets will likely perform best and save money by not buying those that won’t.