Updated: Dec 4, 2022
If the only goal is to hit a ten inch circle at 100 yards, very little attention needs to be paid to the rifle brass. However, as the level of precision increases, so must the attention to detail. One of those details is case length.
Rifle brass needs to be trimmed for ease and safety of chambering, improved precision, and proper depth of cut on the shoulder while neck turning. Ease and safety of chambering is easy to understand. When the rifle case is too long, the neck can be "squished" during chambering. This can lead to hard bolt closing, poor precision, and a multitude of bad things happening to the brass.
Rifle precision, in many ways, is about controlling the variables that can affect the bullet’s trajectory. Neck tension is a piece of the precision puzzle. A smaller, yet still noticeable, variable is the length of the case neck. Case necks that are all the same length, ensure the bullets will travel the same distance through the neck tension upon firing. Resulting in higher precision.
Proper neck thickness is the main goal when neck turning. Albeit, the depth of the cut on the shoulder is also important. This depth is determined when the case neck impacts the stop on the cutting pilot. If the cases are not trimmed to the same length, the depth of cut on the shoulder will vary the same amount. Worse case scenario; cutting too far into the shoulder can lead to case separation at the neck/shoulder junction upon firing.
With all that being explained, case trimming is not rocket science. Anyone can successfully trim brass, as long as a few basic principles are being followed.
Experience Tip: Calipers should be used when measuring case length. Hold the caliper up to a light to ensure the case is square in the jaws. Any angle will show up on the case head and jaw.
When trimmed, the mouth of the case should be parallel with the base of the case. Stated another way, the mouth needs to be cut square. This principle needs to be met if a high degree of precision is desired.
Another basic fundamental is the trimming the brass to the correct length. When using new brass that needs to be neck turned and fireformed, trim the brass slightly short of the maximum case length. I like to trim this brass .002” under max case length. If the brass is already shorter than this, uniform to the shortest piece of new brass. The reason for the “long” length is that new brass routinely shortens when fireformed.
There is no need to uniform new brass that does not need to be neck turned before fireforming. This is assuming all pieces are under the maximum case length to begin with, which new brass usually is. Let the brass move the way it wants during the fireforming then uniform after the second or third firing. For a more detailed explanation of fireforming see Blog titled: Fireforming, "You only get one chance to make good brass."
Experience Tip: There are different ways of determining the maximum case length. If the barrel is custom, ask the gunsmith who chambered it. For factory barrels, the SAMMI specs found in many reloading manuals will work. Chamber casts, when done correctly, can give lots of useful information.
When trimming fired brass, (both competition and hunting brass) I like to follow the
Trim all cases to the same length if any of the case lengths vary by .002” or more.
The #10 actually has two meanings. One; if any of the cases get closer to the maximum case length than .010, I will trim all cases. Two; Trim fired brass to a minimum of .010” from the maximum case length. For example: If the max case length is 1.520”, all cases should be trimmed to or shorter than 1.510”.
.020” from the chamber length is the maximum the fired cases should be trimmed. For example: If the max case length is 1.520” the trimmed case length should be no shorter than 1.500”.
Is the 2-10-20 a "set in stone" rule? Absolutely not. Those numbers have been stolen from some of the best shooters in the world during discussions after matches. I believe the key is not the exact numbers, rather to have a guideline that works and is easy to follow
I like the cases to be trimmed closer to .020” than .010”. During a match or hunt away from home, this extra distance gives a sprinkling of insurance that I will be able to safely chamber the round.
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Uniformity is the name of the game when trimming brass. The higher the desired level of precision, the less deviance there can be in individual case lengths. Few shooters are good enough to notice the difference in groups shot with cases all trimmed .010" from the maximum verses groups shot with cases trimmed at .020". However, shoot a single group with cases that vary in length by .010” and many marksmen can watch their groups open up. If you want a chance at the top rung of the precision ladder, make sure all pieces of brass are exactly the same length.
As long as the operator performs the procedure correctly, the squareness and uniformity of the cut will be dependent on the quality of the trimmer used. Over the years, I have used several different types and styles. My go to trimmer is the Wilson Case Neck Trimmer because of its' consistency.
Experience Tip: When using the Wilson trimmer, the user will feel the cutter "lighten up" when the set case length is reached. Similar to using a skill saw and cutting through a 2x4. The user can feel the resistance of the wood through the saw. Once the wood is gone, so is the resistance. The same feeling will happen when using the Wilson Case Trimmer.
After the cases are trimmed to length, the inside of the case necks need to be chamfered. Chamfering puts an angle to the inside of the case mouth. This angle ensures the mouth does not "grab" the bullet as it is seated. If the angle was not there it could cause the brass to buckle, essentially ruining that piece of brass. For this operation, I use the 45 degree Wilson Uniform Burring tool.
Experience Tip: Excessive chamfering can make the cases grow in length. Using the Wilson Uniform Burring tool along with the Wilson Case Trimmer Stand guards this from happening.
Deburring the outside of the case neck is the last step in case length management. This process removes any burrs left from trimming and chamfering. For this process I use the deburring tool on the RCBS Case Prep Center. A quick pass is then made using the hand held Wilson Case Mouth Burring tool.
Once the brass has been fired, the main reason for a change in case length is improper set up of the resizing die. Specifically, the case shoulders are being bumped too far. For hunting brass, I like to set the shoulder bump to .001-.002”. The shoulder on competition brass is usually pushed back .0005-.001”. For a detailed explanation on how to set up the resizing die see the Blog titled: Reloading Step 1: Resizing the Rifle Case. There is also a YouTube video on this subject.
The importance of case trimming is relative to the level of precision desired. However, do not make this more complicated than what it needs to be. The cases have been successfully trimmed as long as three criteria are met: First, the case mouth is cut square. Second, all cases are trimmed safely under the maximum case length. Third, all cases are the same length. Until next time, enjoy the experience.