The value of the SAAMI and CIP Max Cartridge and Min Chamber prints goes beyond that of chambering firearms and looking at hypothetical bullet – chamber match ups. These prints set the standard for how we determine whether ammunition is fit for the market or is junk. The fastest way to determine if a piece of brass or loaded round is dimensionally correct is the use of a gage. The next few posts will cover the theory, design and usage of gages for a hobby or commercial loader.
Why a Gage?
A gage may be used to check one dimension or many dimensions all at once. Typically in ammunition we want to check to make sure that a case or loaded cartridge is dimensionally correct. We are typical not concerned about just one dimension we are concerned about all of the dimensions, including the location of features to one another.
SAAMI and CIP do not call out tolerances that relate one feature to the next. For example a case could still meet SAAMI Tolerances and have a slightly crooked neck. The neck diameter is called out, and met. The overall length of the case is correct. However if you tried to chamber the round it likely would not chamber as the neck is not properly aligned. This is a inherent flaw in the dimensional callouts that are used in typical +/- tolerancing. This may be fixed with Geometric Dimensional Tolerancing (GD&T) but this is not an industry wide convention.
It is wholly possible to check these dimensions using a Micrometer, Dial Indicators, and other fine measuring devices, but that would be prohibitively time consuming. Additionally it would take skilled labor to take the fine measurements needed, labor that would likely be suited for other tasks. A gage is used as a standard, that is typically quick and simple to use.
To check whether or not a case meets both the dimensional specs called out by SAAMI or CIP and to ensure it will chamber, a set of gages are used. The first gage is the Case Gage which ensures that the brass (also referred to as a case or shell) is made properly. Typically you are checking four main feature which are as follows:
- Does it slide into the case gage?
- Is the case length between the min and max?
- Is the headspace between min and max?
- Is the rim or groove of the case properly formed?
If the case fails any of these checks then it is considered defective. Now you may be able to correct this by running the case through a sizing die, or trimming the die. However this is typically not practical if you are in a production setting. Every additional step you add to the loading process costs money, so you want your brass to come from the supplier defect free. A handloader is more willing to fix cases, to them it is perfectly normal to expect to need to size cases prior to their first loading. This is simply not the case when an ammunition manufacturer orders 100k cases from Starline.
The second type of gage that is commonly used is a cartridge gage. This gage is far more useful to the handloader then the case gage. There are several issues that can affect the dimensional integrity during the loading process. Being overly ambitious on a crimp can bow out the shoulder of case and prevent it from chambering. The loading machine can damage the extractor groove on a case to the point where it may not reliably extract from a rifle. If the bullet is not properly aligned prior to seating it may be seated crooked and refuse to chamber. A cartridge gage is the final gage that ensures a loaded cartridge will chamber in a SAAMI Min spec barrel. Cartridge gages check the following dimensions;
- Will the cartridge chamber?
- Is the cartridge below SAAMI Max COL?
- Does the cartridge sit between Min and Max headspace?
- Is the rim or extraction groove within SAAMI Spec?
Handloaders find the cartridge gage to be more useful when developing loads as it gives a definitive check that the rounds they are loading at the press will chamber in their firearm. Yes, there are ways to check this without an official gage, but usually it involve placing a round in a barrel. This is not a best practice from a safety aspect. For commercial enterprises checking the round in a cartridge gage is the final check before packaging the rounds for shipment. More often than not a cartridge gage is present at the loading machine to do spot checks as the ammunition comes off the loading equipment. This doesn’t replace measuring base dimensions like cartridge COL. However it is the standard by which you can determine if the ammunition is functional or scrap in about 5 seconds.
The Case and Cartridge Gages are important tools in the QA/QC tool box. They are not cheap, they are precision machined gages that are meant to be the final say as to whether or not a case or a cartridge meets the SAAMI or CIP dimensional standard for a given caliber. They are precision tools, and should be treated as such.
The Argument for Building Your Own
There are several case and cartridge gages that can be purchased from reputable reloading supply houses. They are relatively inexpensive, and they do work. However I have some doubts as to how accurate they may be. Are you ensuring that the cartridge meets the SAAMI Max Cartridge Spec, or are they using a SAAMI Min Chamber Spec? They do not advertise to which spec the gage is made to. Nor do you have any type of certificate of conformity that ensures that gage is made correctly. It’s entirely possible that you may get a gage that was made out of spec. In addition the consumer grade gages rarely have any mechanism for checking the rim, or the extraction groove on a case or cartridge.
A precision gage is typically produced by tool and die makers. They are not cheap. You shouldn’t expect to pay less then a few hundred bucks for a quality gage. However if your design is correct, and the gage is made to your prints you can have reasonable confidence that your gage is going to do its job and prevent out of tolerance ammunition from making it to the range. It’s hard to make the case for this type of gage for a hobby reloader. However for a commercial loader, there is considerable more risk in finding that an entire lot of ammunition is out of spec due to a poorly made or worn out gage. $1,000 gage will pay for itself when you have $60,000 in unsellable ammunition due to the absence or use of a $25 dollar cartridge gage.
The Ballistic Assistant will show a few examples of how to take a SAAMI or CIP Max Cartridge and Min Chamber Pit and turn it into a workable case and cartridge gage that can be used to check ammunition check these out below.
Case and Cartridge Gage Walkthroughs