If you want to wring the most out of your rifle, it is important to understand the basics of neck clearance and neck tension.
Terminology
Neck Thickness
Measurement of the width of the case neck. Measured with a ball micrometer.
Over-Bullet-Measurement (O.B.M.)
Measurement from outside to outside of the case neck with a seated bullet. This should be measured when the widest part of the bullet is in the case neck.
Note: This may not be the actual seating depth of the bullet. Measured with a micrometer or calculated.
To keep it simple and avoid miscommunication, I deliberately chose not to use words like pressure ring, heal, shank, bearing surface, etc when talking about the widest part of the bullet.
Neck Clearance
The “gap” between the outside of the case neck (with seated bullet) to the inside of the neck chamber walls. Usually a calculated distance. This number is commonly given as a total measurement however, it can be discussed as “per side.” For example: .002” clearance is .001” per side.
Neck Tension
The amount of “grip” the case neck has on a bullet. Can be measured using a micrometer or calculated.
Neck Clearance
The following numbers can be used as a guideline to ensure proper clearance. A small percentage of rifles may prefer clearances that do not align to this chart.
On the extreme end of the spectrum is negative clearance. This is when the loaded round will not chamber due to the over-bullet-measurement being greater than the chamber measurement.
.001” total clearance or under is generally considered an inadequate amount. This deficiency may lead to hard chamberings and pressure spikes. In the competitive world, there was a time when many thought a total clearance of under .001” was needed for optimal accuracy. Few competitive shooters still follow the “tight neck” philosophy.
Today, the general consensus for proper clearance starts around .0015” and can go to .004”. This is the range that allows for complete release of the bullet yet does not encourage excessive work hardening or blow-back. (In my opinion) there is no valid reason to go much past .002”
On the opposite end of the spectrum is excessive clearance. Too much clearance is usually considered more than .004” total neck clearance. The farther past .004” clearance, the more noticeable the work hardening and loss of precision.
Once proper neck clearance is achieved, uniformity from case to case trumps actual clearance. Meaning, if all the case necks are turned to the same thickness, there should be no noticeable precision difference between .0015 and .0035 clearance.
Experience Tip(s):
A quick way to check if you have enough neck clearance is to take a fired case and insert a bullet nose first. If the bullet is easily inserted, you have enough clearance.
One indication of too much clearance is when the carbon fouling goes way beyond where the bushing resizes.
Mathematical determination of Neck Clearance:
A handloader can find the theoretical neck clearance by performing some basic math:
Step 1: Use a ball micrometer to measure the thickness of the case neck walls.
Step 2: Multiply this number by 2.
Step 3: Measure the widest part of the bullet using a micrometer.
Step 4: Add the numbers from step 2 and step 3 together.
This will be your theoretical over-bullet measurement.
Step 5: Subtract this number from your chamber dimension to find the theoretical
clearance.
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Physically determining Neck Clearance:
To obtain the actual neck clearance, seat a bullet and (using a micrometer) measure the over-bullet measurement. (Remember: The widest part of the bullet needs to be in the neck. This may not be the actual seating depth of the bullet) Subtract this number from the chamber’s neck dimension to obtain the neck clearance.
For example; if the chamber has a .330” neck and the loaded round has an over-bullet-measurement of .327” then there would be .003” of clearance. Or…0015” clearance on each side.
If physically determining neck clearance is easier and has a higher chance for accuracy, why would anyone want to do it mathematically? Answer: reloaders who turn their necks. The handloader can decide what clearance they desire, then “reverse math” to determine the thickness of the case neck.
For example; if a shooter has a .330” chamber and wanted to have .002” clearance they could figure out their neck thickness by the following process.
Step 1: Subtract desired clearance from neck chamber dimension
.330 - .002 = .328
Step 2: Subtract widest part of the bullet measurement from the answer in step 1
.328 - .3086 = .0194”
Step 3: Divide that answer by 2
.0194/2 = .0097”
For this example, .0097 is the thickness the case neck walls should be shooting the bullet we measured in step 2. Different bullets can have an influence on neck clearance.
Experience Tip:
We can easily calculate to the nearest .0001”. Realistically, setting the turner up to this measurement is not quite that easy and turning each neck to that degree of uniformity is even more difficult.
After turning the first case, I measure the neck thickness and physically check the clearance before turning all the pieces of brass. Once I get rolling, I will measure the neck thickness every ten cases or so.
Neck Tension
Neck tension is a small, yet influential, piece of the tuning puzzle. Handloaders have control over this variable, when they understand the important relationship between neck bushings and the over-bullet-measurement.
Mathematical Determination of Neck Tension:
A common way of finding the neck tension is to take the over-bullet measurement and subtract the number printed on the bushing. For most rifle applications, this method is perfectly adequate.
Physically determining Neck Tension:
If you are searching for the utmost in precision, you may want to consider measuring the neck tension. I do not own a seating gauge so I use this method taught to me by a Hall of Fame shooter. First resize the case neck using the desired busing. Using a micrometer, measure the middle of the resized portion of the case neck.
Next, using the same case, seat a bullet and measure the over-bullet diameter. Subtract the resized case neck dimension from the over-bullet measurement to obtain the actual neck tension.
For example; if the over-bullet measurement is .3274” and the resized neck measures .3236” then you are using .0038 neck tension. For ease of discussion, this would be rounded to .004.
Reason for doing this: Bushings do not always resize to the dimensions printed. (There could be several reasons) This may be important to know depending on the level of precision required. For example;
pictured is a resized neck using a .324 bushing. I have a .323 bushing that resizes annealed brass to .3234. I do not know anyone who can shoot the difference in .0002 neck tension. Yet, the bushings indicate a change in neck tension by .001.
In hunting rifles, I usually run .002 to .003 neck tension. In competition rifles, I'm consistently in the .003. to .004 range. As always, the target will tell me what tension the rifle likes.
Neck Clearance's Influence on Neck Tension:
Over time, neck clearance will have an influence on neck tension. The farther the walls of the neck have to move during sizing and firing, the faster the negative effects of work hardening will occur. Work hardened brass will not "move" the way new brass does, therefore impacting neck tension. The shooter has three main ways of combating this issue:
Properly anneal the brass
Adjust the bushing to maintain tune
Quit using work hardened brass and use new (fresh) brass.
Common Fault:
A common fault on the internet is for people to only share the bushing size when discussing neck tension. That is only half the equation. To be able to figure out how much neck tension is being used, the neck bushing (or resized neck dimension) and the over-bullet-measurement has to be given.
For example: A hot shooter posts that he uses a .324 bushing in his 30 BR, but does not give any other information. People reading his post may assume they should switch to a .324 bushing. However, because the over-bullet-measurement was not indicated, we do not know what is truly important; neck tension.
Simply better would be to state the actual neck tension used. The reloader can then use the information in this Blog to determine what bushing they should use in their own cases using their own bullets.
If you are searching for the utmost in accuracy and precision it is important to know the definitions, how to measure, and understand the relationship between neck thickness, over-bullet-measurement, neck clearance and neck tension. Until next time, enjoy the process.
