Below are notes on tools and techniques you may find useful for recording the physical characteristics of your components.
Many of our components, particularly powder, bullets and primers come in packages marked with a ‘lot number’. The lot number is a quality control number assigned at the factory. Its purpose is to assure the product has been manufactured to specifications, and to permit targeted recall if it is later determined there was a problem. One assumes the entire batch that makes the ‘lot’ to have been manufactured in one pass, thus the individual items are as close to uniform as possible.
Powder in particular is likely to vary somewhat between lot numbers, and unless you specifically purchase either large quantities, or many small units of the same lot number, you probably will be testing that statement regularly. Loads which approach maximum pressure should be tested and adjusted if necessary when the lot number of your supply changes. It is not a good idea, and is not recommended that you mix powder from differing lot numbers for this reason.
Among the various components we use, powder is the one item we can not test prior to using it. Prudence suggests you always ‘work up’ to a load, and do not assume a new lot of powder will perform exactly the same as a previous lot.
Cartridge cases for commercial ammunition have drawings on file with SAAMI.
Step one is to retrieve the drawing for your cartridge from SAAMI and look it over.
Compare your cases to the drawing for the following dimensions:
Gutentor Simple Text
Gutentor Simple Text
Gutentor Simple Text
Point your web browser to http://SAAMI.org/ and use the menu system to select “Technical Information“, “Cartridge & Chamber Drawings“, then select from “Rimfire“, “Pistol and Revolver“, “Rifle” or “Shotshell“, to load a very large PDF containing all of the current accepted cartridges of the selected type.
You can use the index to locate a page number, or you can use your browser’s ‘FIND’ or ‘Search” function to look for the name of your cartridge. We will use “Rifle” and “6.5 Creedmoor” in this example. The index says the drawing is on page 39, so we scroll down, while watching the page numbers at the bottom of each page (ignore any browser generated page numbers – you want the printed page number from the PDF).
Once page 39 is located, in the upper right corner you will find the cartridge name – typically in blue on teal. Check to make certain this name matches the cartridge you are working with. There are some ‘tricky’ names, as the guys in marketing at the manufacturer are the ones who give cartridges their ‘official name’.
In the top half of the page you will find the cartridge drawing and the lower half will be the chamber drawing.
Looking at the cartridge drawing above, you will see lines, arrows, numbers and brackets as well as symbols and text.
The cartridge is shown as an outline, and lines that have arrow heads show dimensions across the outline – diameters. Lines without arrow heads indicate starting and ending points of dimensions along the length of the cartridge, and the lines within those have arrow heads pointing to the start-stop lines to indicate the measurement between those points.
The measurements are in primary units of inches and secondary units of millimeters. The metric units are enclosed in square brackets for example the dimension immediately below the case body is 1.150 inches or [29.21] millimeters and is marked with the letter ‘B’ which you can see in the notes means ‘Basic’. The lines pointing toward the case have arrowheads and align with another similar line on the opposite side of the case. This set of lines indicates a diameter .4703 [11.946] at the head and .4629 [11.758] at the shoulder. So the takeaway from this dimension is that it is a coordinated diameter and length, that establishes the ‘slope’ of the case body.
You will also note triangles and stars used to call out some measurements. At the bottom of the page you will see that ‘circle x’ is a HEADSPACE, ‘triangle’ is a REFERENCE, ‘asterisk’ is a dimension to the intersection of lines.
In this drawing, the notes section in the lower right corner specifies the basic tolerance. In certain measurements for example, in the case drawing, case length the dimension is written: 1.920-.020 [48.77-0.51] which specifies the maximum case length at 1.920 and the minimum case length at 0.020 shorter or 1.900. Looking at the chamber drawing the matching dimension (bolt face to the point where case neck diameter has reduced from .2970 at the neck shoulder junction to the diameter at the mouth of .2960) the length is 1.925 and is taken as the end of the neck portion of the chamber. A 45
° slope connects to end of the chamber to the beginning of the leade or freebore at a diameter of .2645.
The important thing to understand about case neck length is the coordination between the case dimensions and the chamber dimensions. In the case of the 6.5 Creedmoor, the maximum case length of 1.920-0.020 and the minimum chamber length is 1.925+0.015. There will always be at a minimum 0.020 inches of clearance between the longest case and the shortest chamber. There could be as much as 0.035 inches clearance if tolerances stack upto maximum.
Another critical dimension is called ‘Headspace’ it is the ‘circle x’ call out, and it’s length is 1.551 MAX to 1.541 MIN. Headspace is taken from the bolt face to the datum point on the shoulder of the case. Datum is defined by a diameter along the slope from the case-shoulder junction to the shoulder-neck junction. Chamber datum is 0.400, and case datum is identical. Case length CBTD (Case Base To Datum) is 1.5438-.0070 [39213-0178]. The minimum CBTD is 1.5368, maximum is 1.5438. The maximum length case in the minimum length chamber leaves a difference of 0.0072, while the minimum case length in the maximum chamber length is 0.0042. The difference between the two is 0.003 and is the actual ‘head’ ‘space’, or distance the design allows the head of the case to expand under pressure.
Springback is a property of brass that causes it to contract slightly as it cools. For typical cartridges the amount of springback is assumed to be about 0.002. Thus a fired case measuring 1.5380 would have been 1.540 at the point of maximum pressure, and that gives us a good approximation of the chamber dimension.
Resizing this fired case to 1.536 bumping the shoulder back 0.002 from its fired length, or approximately 0.004 from its maximum length under pressure should give the case plenty of operational clearance, while maintaining a slightly tight headspace very near the minimum end of the 0.0042 – 0.0072 design range.