The Over Bullet Measurement (O.B.M.) is the dimension from outside to outside of the case neck with a seated bullet. It is an important measurement in several fundamental numbers used when reloading. The two most popular are neck clearance and neck tension.
A common mistake when finding the O.B.M. is measuring the case neck in a location other than over the widest part of the bullet. This produces a lower/incorrect O.B.M. The end result is a false increase of clearance and a mathematically reduced neck tension.
Step 1: Resize the case neck with provided expandable ball or desired bushing.
Step 2: Using a micrometer, locate the widest part of the bullet you are using. Realize not all bullets have a pressure ring.
Step 3: Seat the bullet, stopping when the widest part is half way in the resized portion of the neck.
Important: This may not be the final seating depth of the bullet. This process is done solely for finding the O.B. M and should be completed before load development begins.
In the picture to the right, the 112-7 grain BIB bullet has a pressure ring (widest part) located at the heel of the bullet. In this example, the heel is seated halfway in the resized portion of the neck.
Step 4: Measure the outside to outside dimension at the middle of the resized neck. This is the O.B.M. for that batch of cases and bullets.
Safety note: When changing brands/lots of cases or bullets, the OBM should be rechecked.
Experience Tip: There are some machinists and gunsmiths talented enough to perform this analysis with a caliper. For the rest of us mortals, the O.B.M. measurement should be executed with a micrometer.
Why this needs to be Correct
The O.B.M. will measure a smaller diameter when the neck is measured in a place other than over the widest part of the bullet. This usually occurs because the widest part was seated below the resized portion of the neck. When the round is fired, the widest part of the bullet has to “re-enter” the smaller diameter. The result is a decrease in clearance and an increase in neck tension.
Neck Clearance Mathematical Example; if the correct O.B.M. is .328” and the chamber neck dimension is .330” that particular round would have an actual clearance of (.330 - .328) .002”. For example purposes; let's use an incorrect O.B.M. measurement of .327” This would give a false clearance of .003”
Neck Tension Mathematical Example: if the correct OBM is .328 and the resized neck dimension is .324 that particular round would have (.328 - .324) .004” of neck tension.
Staying with the same example, an incorrect .327 O.B.M. would calculate to .003” of neck tension.
Why this Matters
1. The increase in neck tension and the decrease in clearance may be a non issue or it may increase pressure to a dangerous level. For most hunters, using .003” or more clearance, this mistake would probably never be known. However, the same is NOT true for competition shooters running a tight neck of .001” clearance. The degree of pressure increase is dependent on many variables that will be unique to each situation.
2. Finding the correct theoretical O.B.M. is a critical aspect of determining neck thickness when neck turning.
3. Repeatability of neck tension when changing brand/lots of brass and/or bullets.
4. Ensuring proper neck clearance when changing brands/lots of brass and/or bullets.
5. Correct information is important when discussing neck tension and neck clearance with others.
Experience Tip: .002” is considered the “ideal” amount of clearance. No difference on target should be noticed if the clearance is in the .0015 to .003 range. Clearance below .0015 may create dangerous pressures, especially with an incorrect O.B.M. Loss in precision and negative effects of work hardening will coincide with clearance greater than .003”.
Experience Tip: .003” neck tension is a good place to start load development. The rifle will tell you what neck tension it likes by the group sizes and shapes. Diagonal groups may indicate a different neck tension is needed.
For more in-depth information on these topics see the Video/Blog: Neck Clearance and Neck Tension.
Details matter. Over Bullet Measurement is one of those details. To get this right, measure when the widest part of the bullet is in the resized portion of the neck. Until next time, enjoy the experience.