Educational Zone #92 – How to Trim Rifle Brass

I have shot probably hundreds of thousands of rounds of pistol ammo, and reloaded a lot of it too. But if you only shoot pistol ammo, you do not really need to be concerned about the brass stretching and getting too long. But with bottle-necked rifle ammo, this is a different story.

When bottle-necked rifle rounds are fired, the brass tends to stretch and the brass gets a little longer each time it is shot and resized and reloaded. Eventually, the brass could get so long that the neck of the brass will reach all the way forward until it can reach the beginning of the rifling. If it does this, it can “pinch” the bullet in the case and cause very high pressures and even case or primer failure.

Therefore, bottle-necked rifle brass must be trimmed to a safe length.

How do we know when this is needed? That is easy. We refer to a chart that lists maximum and minimum case lengths and use a micrometer to measure the length of our cases.

Today, I had some .30-06 brass to reload. I started off by neck-sizing the brass and de-priming it. Then I checked the length.

The chart shows the maximum length to be 2.494 inches and the “trim to” length to be 2.484.

I do not believe the “trim to” length is extremely important, it is just important to trim it below the maximum length.

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My cases averaged over the 2.494 ,maximum, so it is time to trim them.

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So, how do we go about this task?

There are many different ways to do the job, but I will show you how I do it.

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I have a Lyman Universal Drill Press Trimmer base attached to a piece of ¾ inch plywood that I can mount on my drill press.

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Here it is.

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This base has two flanges that fit over the rim of the rifle case.

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You can then twist it clockwise, and it will lock the piece of brass very solidly in place.

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To trim the brass, I use the other half of this system, the trimmer.

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It has different sized “buttons”, one for each caliber.

I installed the .30 caliber button today.

The four sharp stainless blades will trim the brass.

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I mount it in the drill press head and adjust it to fit exactly inside the case mouth and lock it down solidly.

Then I adjust it until it will cut just a little of the brass off the case.

Trial and error will get us to the place where it trims them to at least 2.484 or less.

Once adjusted, I lock it down and am ready to process all the cases.

I use the slowest speed on my drill press, as it is plenty fast enough for these operations and there is no big hurry.

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Since this set-up is a little trouble, I usually trim at least 50 or 100 cases at a time.

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Here are some examples of the brass that is removed from the cases.

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You just place a round of brass in the base, and pull does on the drill press handle to trim it to length.

Slow and steady does a neat job.

Here’s what the mouth of the cases look like after trimming.

As you can see, it leaves a flat end and also a ring of surplus brass on the inside and outside of the case mouth.

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Sometimes you will also see some loose brass on the mouth of the case.

We will need to trim this off.

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To do this task, I use an RCBS Case De-Burring Tool.

It has cutters for both inside and outside the case mouth.

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I also have a mount for it that fits the drill press to help with the operation.

You just loosen the set screw and insert the de-burring tool in the mount.

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I set up the de-burring tool for outside trimming and run each piece of brass through it.

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This removes the ring of brass from the outside of the case mouth and makes it nice and smooth.

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Then I loosen the set screw and reverse the tool to do the inside of the brass.

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Once again, I run each piece of brass through the operation.

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You will pick a lot of brass shavings on this tool also.

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If the brass is once-fired military brass, I can also use this tool to remove the primer crimp from each case.

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Then I measure the brass and find that it is all below max trim length, but it is also all uniform in length.

In my opinion, “Uniformity is the Key to Accuracy” in reloading.

The more uniform each round, the more likely that they will go into the same hole in the target.

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Finally, I have all the brass trimmed and it is ready to go into the vibrator/polisher to remove any brass chips or other foreign matter from the cases.

It is then ready to prime and reload.

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Conclusion:
Is this a great hobby or what!