This is the first in a series of write up detailing the design of cartridge and case gages for various cartridge types. If you are not familiar with how different cartridges headspace, please check out this write up.
Cartridges that headspace off the case mouth require only one gage, a cartridge gage. While a separate case gage can be made, a cartridge gage is sufficient to check both the headspace, the case length. It will also check to make sure that a loaded cartridge will chamber in a SAAMI or CIP min spec chamber. Since all the gages follow a similar set up and geometry, detailing the design of this gage is a good introduction to the design of both cartridge and case gages.
External Dimensions and Material
Shown above is the basic external geometry of a case gage. Blanks are typically made from 1.250in 17-4 Stainless. These gauges are handled day in and day out for the period of a few days and then stored until needed again. For some reason the oils on some people’s skin are particularly corrosive, and if not properly cleaned and oiled prior to storage, they can cause corrosion.
The blank is machine to the max cartridge length. In this case for 30 Carbine it is 1.680in. Typically the ends of the stock are machined down to 1.00in diameter. This is wholly optional.
Max Cartridge Geometry
A cartridge gage is a combination of Max Case Geometry and Min Chamber Geometry. As you work through the sketch of the Max Cartridge Geometry often times you can learn things about the caliber that you would never have learned otherwise. 30 Carbine happens to be a good example of this. In order turn a max cartridge drawing into a gage, we are creating a “chamber” that is at max cartridge dimensions.
When converting a SAAMI or CIP Max Cartridge Drawing to a Cartridge Gage drawing, it is typically started from the left. The mix thickness of the rim of the case, in this case is 0.050in. The diameter is 0.360in. SAAMI defines the taper on a case body using a basic length and diameter. This is a SAAMI standard convention that you will find on all SAAMI Drawings.
The next line that is drawn is the case body. This line is defined by two points. One at 0.200in from the breech, with a diameter of 0.3548 and a second at 0.6700in and a diameter of .3387. A line is drawn through these two points to define the case body. The line is extend back towards the case head to meet up with the rim of the case. This is where is can be interesting, and where we learn something about the 30 Carbine Case.
As seen in the image above when the line is extended from the case body to meet with the rim of the case the line does not intersect the point at the edge of the rim, rather it would intersect in the middle of the rim. In these instances, a micro step is formed as shown above. In theory, a 30 Carbine case can have a rim .0008in larger than the case body, in reality this is unlikely to be encountered. Why include the step if it’s inconsequential? Well, the argument is that this is drawn correctly with a step, if we fudge it a little then we have a gage that is pretty close, but not exact.
The 30 Carbine case tapers down to the case neck, much like a bottle neck case. There is a long straight case neck, and then the end of the case. Max SAAMI Case length is 1.290in. While the max headspace is 1.290 to 1.300. SAAMI intends that a Max Cartridge will have 0.00 to 0.010in clearance on the headspace. When we check a headspace on the cartridge we are checking it according to the Max Cartridge dimension to preserve the intended min-max clearance between a Max Cartridge and a Min Chamber.
Once the max cartridge drawing is drawn to the end of the cartridge, the min chamber is used. In this case lead in and bore are all cut into the gage. The highlighted dimension in the image below are all taken from the Min Chamber Drawing.
As previously mentioned overall length of the gage is the Max Cartridge Length, in this case it is 1.680in. The min chamber checks to make sure the bullet is centered in the case as to chamber freely. If the bullet is crooked or not centered in the cartridge, this can cause the bullet to be off centered and rub against the lead in.
Once the geometry is modeled correctly, it looks like this.
The Go & No-Go Gages
In order to check the overall cartridge length and the cartridge headspace/case length two steps need to be cut in either end of the gage. The depths of these steps are taken directly from the Max Cartridge drawing. This is shown in the image below:
The 30 Carbine case length is 1.290-0.010in and the Max Cartridge Length is 1.625in to 1.680in. A 0.010in step is cut on the left side of the gage to correspond with the case length/headspace, while a 0.055in is cut into the right side of the gage to correspond with SAAMI Max Cartridge.
Finishing Touches
Now that the internal of the gage is finished, a finishing touch may be applied to help identify the gage, and to help prevent it from rolling away.
A simple flat is cut on one side of the gage and the caliber, and gage type is labeled. When the gage is set down on the desk, it is less likely to roll away, and when you pick it up from the tool cabinet you can be reasonably assured you have the right gage.
Use of the Gage
The gage as currently designed will check 3 out of the 4 measurements. Namely, headspace/case length, cartridge overall length, and does it chamber?. This is a functional gage. The use of the gage is simple a list of steps is provided below.
- Check to make sure the gage is clean and free of debris. Dust or debris can affect the gage’s ability to provide meaningful measurements.
- While holding the case horizontal slip a case or a cartridge into the left side of the case.
- Tip up the gage vertically and allow the cartridge to freely drop. The rim of the case should fall between the high and low step. If the rim sits proud of the No-go step, the case length is too long, if it sits below the Go Step, the case length is too short.
- If the cartridge is loaded, the nose of the bullet should be visible and fall between the Go, and No-Go step. If it is above the No-Go step, the cartridge is too long. If it is below the Go step, the cartridge is too short.
- If you suspect the case is not seated in the gage it can be tempting to try and force it with your thumb. DO NOT do this. Hold the gage with the case/cartridge in you hand. Use the forefinger of the hand holding the gage to apply pressure to the case and see if it will seat. Applying pressure in this manner limits the amount of pressure that can be applied and ensures you are not getting a false reading.
- Before putting the gage away, wipe it clean, inside and out.
Checking the Rim and Extraction Groove
The last step of designing a complete gage is to add the means to check the rim and extraction groove dimensions. This dimensions are to check and make sure the geometry doesn’t exceed the maximum dimensions allowed by SAAMI.
When modeling the case head be sure to use the dimensions at the max material condition. So if it’s 35deg + 20deg use the 35 deg. The gage is cut as a slot through the top of the gage as shown below.
Use of the Case Head Gage
A case head that is below the SAAMI Max Cartridge should slide through the gage without a problem. If there are dimensions that exceed SAAMI Max the case should not slide through and should not be forced through the slot.
Tolerances
Since this is a gage, the tolerances are extremely tight and will add considerable expense to the gage construction. Typically all diameters are +0.0001 to +.0005 all lengths are similarly called out at +0.0001 to +0.0005.
Drawing Example
The Ballistic Assistant is working on putting together a drawing for this cartridge gage and will upload it when it is available.