By Mark V. Lonsdale
As a continuation to the series of reloading fundamentals for rookies, we will take a look at the importance of measuring headspace and accurately bumping the shoulder on your brass. But keep in mind that this article is a “why to” not a “how to.” The how to will come in the instructions with whichever measuring system you decide to go with, plus a reputable reloading manual.
The tool to measure the headspace or shoulder position markets under a number of names – shoulder bump gauge, precision mic, or cartridge comparator – but each does basically the same thing. They give a measurement from the base of the cartridge to a predetermined spot on the shoulder. So, why do you need to know that?
A comparator allows the shooter or reloader to do a number of things:
- Measure the headspace of factory or reloaded ammunition to ensure it is within spec for safe and reliable use. If the case is too long it simply will not chamber, and if it is too short, it will not headspace correctly, may not fire, or may result in a case separation.
- Quantifies the chamber headspace in a particular rifle by measuring the headspace of fired cases from that chamber. After firing, a cartridge fire-forms to the size of the chamber.
- It will ensure that you are getting the minimal shoulder set-back when setting up your reloading dies.
This last one is probably the most important for the reloader. By minimizing the set-back of the shoulder, you are limiting the expansion or stretching of the case during firing, improving consistency and accuracy, and extending the life of the brass. So the cartridge comparator also serves as a diagnostic tool to ensure the rifle chamber is within specifications, and to gauge if the sizing die is set-up correctly.
As an example, if you measure the shoulder on once-fired brass from a rifle chamber that is reamed to spec, you will find that the headspace will measure “0” on an RCBS Precision Mic gauge. This is because the case has fire-formed to the length of the chamber.
New factory match ammunition may read minus three on the same gauge. In other words, the shoulder is three thousandths (0.003”) shorter than the spec chamber. This is so that it will chamber correctly. After firing, that case will be fire-formed to the chamber so should have moved the shoulder forward to match the actual size of the chamber.
Now, after you have run a fired piece of brass through your sizing die you may find that you have pushed the shoulder back ten to twenty thousandths (0.010” to 0.020”). This is too much. So now you can adjust the sizing die up to bump the shoulder back just enough to chamber in that particular rifle without any issues. Once you are getting the required amount of bump on your bump gauge, you can lock the die.
Whidden shoulder bump gauge inserted into dial calibers
So for optimum accuracy and case life, the goal is to set the shoulder back the minimal amount while still enjoying reliable feeding. But if you own several rifles of the same caliber, and you want your reloads to function flawlessly in all your rifles, then you will need to set the shoulder back from the shortest chamber. This way it will cycle in all rifles.
Hornady collet system
There are a number of options for measuring headspace and shoulder set-back.
- One of the more common methods, and least expensive is a collet or bump gauge that fits in a standard set of dial or digital calibers. Hornady and Sinclair are popular and the collets are quite affordable. The most expensive part is an accurate set of calipers, which every conscientious reloader should own.
- Another option is the RCBC Precision Mic set specific to each common caliber. These are very handy but a little slower than using calipers. Cost is around $50 depending on caliber.
- The top shelf option is a Cartridge Comparator from Dave Manson Precision Reamers. This system consists of an indicator stand with a base, datum blocks, and plunger-type dial indicator. Once set-up it is very accurate and efficient to use. Check out http://www.mansonreamers.com
RCBS Precision Mic system
Dave Manson Cartridge Comparator dial indicator stand
These three systems are not only used to measure headspace and shoulder set-back, they are also be used to measure overall length and seating depth for bullets. For more consistent accuracy, you will generally want to load your bullets longer and closer to the lands than most factory ammunition. You will often hear shooters talking about 0.015” off the lands or a jump of twenty thou – meaning that the ogive of the bullet is 0.020” back from the lands. However, if you are feeding rounds from the magazine for hunting or competition, then you will be limited to an overall length (OAL) that still fits in the magazine.
Finally, as you get into reloading, and in particular reloading for precision rifle shooting, the more you need to quantify actual measurements and tolerances. Don’t think of this as a painful chore. Once you really understand the advantages of reloading, and see the proof on the target in your scores and the tight groups, you will find precision reloading to be as enjoyable as the shooting (well, almost). There is something very satisfying about producing high quality rifle ammunition.
For additional reading, study the introductory chapters in the reloading manuals from Berger, Sierra, Hornady or Speer. There are also some excellent books such as Handloading for Competition; Extreme Rifle Accuracy; or Prone and Long Range Rifle Shooting.
On left, 308 Win with the Berger 185 grain Juggernaut loaded long. On the right, 375 CheyTac using the Cutting Edge 352 grain bullet. Both loaded into Peterson match grade brass