Hornady makes a simple and very useful tool for the rifle ammo reloader looking to improve accuracy. It is call the Hornady OverAll Length (OAL) Gauge. With this tool, it is possible to find out where the rifling in the rifle begins. Using this information, the reloader can seat the bullet to chamber very close to the rifling. A few thousands of an inch change between the bullet and the rifling can go a long way in improving accuracy. Hornady's direction / documentation is very good.
The OAL gauge comes in 2 versions - straight (for bolt action rifles) and curved (for semi-auto rifles). To use the OAL gauge a Modified Case needs to be purchased separately and is sepecific to the caliber of the rifle. The Modified Case is threaded onto the OAL gauge.
Pictured below are the components (curved OAL gauge, Modified Case & a .224 bullet) needed to use the OAL Gauge.
This picture shows the components assembled ready for use.
Steps:
1. Thread the Modified case onto the OAL gauge and place a bullet into the case mouth. It will and should be loose.
2. With the bolt removed (or held back), place the OAL gauge assembly into the chamber. Be sure the fit is snug.
3. Carefully slide the gauge's internal push rod forward until the bullet makes contact with the lands. Tighten the brass thrumbscrew. This will lock the push rod and bullet in place. Remove the unit from the rifle. The bullet may fall out but can be easily inserted back into the gauge as the thumb screw is holding the push rod in place.
4. Position the caliper in the measuring port on the gauge. This port allows the caliper blade to rest on the case head providing relatively easy measurement of the longest possible bullet/case combination that will fit into the rifle chamber.
The reason I stated that this provides a "relatively easy measurement" is that measuring across bullets tips is very challenging and not a reliable method for measuring loaded rounds. The reason is simply that there are variations in bullet tips. Hornady has a very good tool to solve the problem. It is called the Hornady Bullet Comparator. It consists of a Body and a Bushing insert that is caliper specific. The Comparator comes with 6 inserts for the most common rifle calipers. The Body (and Insert) are attached to the traveling blade of the caliper. See picture below.
The Bullet Comparator measures from a reliable surface -- the bullet ogive to give consistent and precise measurements of the rounds. Set up is easy. 1. Close the caliper blades snugly on the Bushing Insert and Body. 2. Tighten the Thumb screw and the Set screw. 3. Zero the caliper by setting the dial to "0" to achieve the same starting point in future measurements.
To take a reading, open the caliper blades, center the case head on the caliper's fixed blade and slowly close the caliper so that the bullet nose feeds into the Comparator until contact is made. Be certain all surfaces are aligned and all slack is removed as light pressure is applied to the caliper blades.
Note the reading and take 5 to 10 more readings. There will be some slight variations in the number after each fit. Remember that we're dealing with thousands of an inch. Usually there is two or more identical (or very close) readings and that is the number I usually go with.
For example: Using the Hornaday OAL gauge on my Windam Weaponry, AR-15 with a 73 grain Burger Bullet, I got the following readings: 1.940, 1.932, 1.876, 1.868, 1.872, 1.869, 1.875. I went with 1.875. Subtracting .040", results in 1.835. After adjusting my Redding Seater die I had bullets seated at 1.812 thru 1.827 -- close to 1.835.
In the details directions, Hornaday recommends a free-travel (clearance) of .020" to .040" between the bullet and rifling lands for hunting rounds. They state that free travel over .040" will most often result in a loss of accuracy. However, each gun will have its own preference, and is up to the reloader to determine the optimum bullet free travel by range testing the loads. I decided to range test the bullets before I made any further adjustments in my seating die.
Field Test
Shot 10 rounds at 100 yds with the 73 grain Berger bullets resulted in two bull eyes and the remaining 8 within a three inch circle. I will raise the seating die just a bit to get closer to the ideal 1.835, then test again.
Shot 10 rounds at 100 yds with the 55 grain Hornady bullets and got slightly better results.
Both Hornady tools are reasonably priced and readily available from on-line distributors such as MidwayUsa.com and Opticsplanet.com.